In a medium bowl, combine the smoked paprika, dried oregano, ground cumin, salt, garlic powder, lime zest, lime juice, and olive oil. Whisk together until evenly combined — the marinade should look like a uniform, fragrant spiced oil with the lime juice distributed throughout. The spice blend is identical to the chicken and steak fajita versions — the same smoked paprika, cumin, and lime profile that defines fajita character across all three proteins. The only meaningful difference is the marinating time, and this is not a minor detail: the marinating window for shrimp is 15–30 minutes at room temperature only, and this limit is strictly functional rather than a suggestion. Shrimp flesh is composed of loosely structured muscle proteins that are significantly more delicate than chicken or beef. The lime juice's citric acid begins denaturing — effectively cooking — these proteins almost immediately on contact. At 15 minutes the acid has lightly penetrated the surface and the spices have begun to adhere. At 30 minutes the acid has worked further and the shrimp is at the maximum marinating benefit. Beyond 30 minutes, the acid continues cooking the proteins progressively — the shrimp's surface begins to turn white and opaque as if heat-cooked, the texture tightens and becomes rubbery, and by 60 minutes the shrimp is essentially ceviche without ever having touched a pan. Add the shrimp to the marinade and toss thoroughly to coat every surface. Leave at room temperature for 15–30 minutes.

Peel the white onions and cut the top and root ends off each one. Halve each onion from root to tip, then place each half flat on the cutting board and slice vertically from top to bottom following the natural lines of the onion's layers — producing long, thin feather-shaped strips rather than rings. The vertical feather cut is the correct technique for fajita vegetables: the strips char at their thin ends while remaining juicy in the thicker middle sections, and they collapse and weave together with the pepper strips in the finished dish rather than sitting as separate rigid pieces. Combine all three onions in a large bowl after slicing.

Cut the top and bottom from each bell pepper. Stand each upright and slice the four curved walls of flesh away from the central seed column, producing four flat pieces per pepper. Trim any remaining white pith or seed membrane from the inside of each wall piece — the pith contributes bitterness and should be removed. Slice each flat wall piece into strips of consistent thickness matching the onion feathers, approximately 5–7mm wide. Slice the bottom pieces the same way, removing any seed attachment. Mix all sliced pepper strips with the onion feathers and toss together. The three-colour combination is a deliberate flavour decision: green peppers bring a slightly bitter, more vegetal note from being the least ripe; yellow peppers bring a mild, moderate sweetness; red peppers bring the highest sugar content and the most pronounced caramelised sweetness when charred. All three together produce a more complex, layered vegetable component than any single colour could achieve.

Heat a large cast iron skillet, carbon steel pan, or stainless steel skillet over high heat for 2–3 minutes until genuinely hot and the oil added to the surface shimmers and begins to smoke at the edges. Cast iron is the preferred material because its heat retention maintains the pan temperature through the brief contact time of each shrimp batch — a temperature drop during shrimp cooking means the shrimp begins to steam in its own moisture rather than searing. Lay the shrimp in a single layer without crowding — each shrimp needs its own space on the pan surface to sear rather than steam against its neighbours. Work in two or three batches without hesitation. The timing from this point forward is the single most critical technique in the entire recipe: sear each shrimp for 30–45 seconds on the first side without moving. The shrimp is ready to flip when the underside has turned pink from grey and a faint golden-brown sear mark is visible at the contact points with the pan. Flip each shrimp and cook the second side for 30–45 seconds. Remove from the pan immediately when the shrimp has turned completely pink and opaque with just the faintest translucency remaining at the very thickest point of the body. This last fraction of translucency is the safety margin — the residual heat in the shrimp will continue cooking it for another 30–60 seconds after it leaves the pan, and pulling at just-opaque produces a shrimp that is perfectly cooked through by the time it is served. Pulling at fully opaque throughout produces a shrimp that is slightly over-cooked and beginning to tighten. Pulling at still-grey and translucent produces one that is under-cooked. Transfer cooked shrimp immediately to a bowl — do not leave them in the hot pan even for 30 seconds, as the residual heat of the cast iron will continue cooking them past the ideal point. Repeat with remaining shrimp.

Without cleaning the pan — the residual shrimp marinade fond caramelised onto the surface is concentrated spiced flavour — add the sliced pepper and onion mixture. The moisture released by the vegetables as they begin to cook will deglaze the pan and lift every caramelised bit of the fond, incorporating it into the vegetable mixture. This is the reason the vegetables always follow the protein in the same pan: all the flavour left behind by the shrimp transfers to the vegetables. Stir frequently over high heat for 6–8 minutes. You can and should allow the vegetables to char more aggressively than feels intuitively correct — push the onion feather tips and pepper edges to genuinely dark spots. When the vegetables are subsequently combined with the shrimp and rest together, any accumulated moisture and steam softens and distributes the char character throughout the mixture, lightening the intensity at any single point and spreading it as a background smokiness across the entire dish. Work in batches if the pan is not large enough to cook all the vegetables simultaneously — a crowded pan steams rather than chars. If any marinade remains in the shrimp bowl, pour it over the vegetables — it will sizzle, caramelise, and contribute its full spice profile to the vegetable char.

When all the vegetables are cooked, transfer them to the bowl with the cooked shrimp. Toss everything together once to combine — the shrimp juices and the vegetable fond will mix into a light, unified sauce that coats every component. Cover loosely and allow to rest for 5 minutes before serving. The rest allows the temperatures to equalise between the hot vegetables and the cooled shrimp, the juices to redistribute, and all the flavour elements — spiced marinade, caramelised vegetable fond, lime, and shrimp's own natural sweetness — to integrate into a cohesive, unified fajita rather than separately-cooked components placed together.