Set your oven to the broiler setting on its highest heat and position the rack as close to the broiler element as possible — approximately 10cm below it. While the broiler heats, prepare the vegetables. Halve the Roma tomatoes through the equator. Roma tomatoes are specified because their meaty, low-moisture flesh concentrates beautifully under the broiler — their lower water content means they char and caramelise at the surface rather than releasing liquid that prevents browning. Standard round tomatoes work but produce a wetter, less concentrated salsa. Peel the garlic cloves and lightly smash each one with the flat side of a knife — a single firm press that cracks each clove open without fully flattening it. Peel the white onion, halve it through the root, then cut each half into 4 even wedges, keeping the root intact on each wedge so the layers hold together during broiling rather than falling apart into individual rings. Place all three vegetables on a large rimmed baking sheet. Drizzle the olive oil over everything, scatter the ground cumin across the vegetables, and toss everything directly on the tray until evenly coated in oil. The cumin blooms in the olive oil during the toss and distributes more evenly across the vegetables than if it were added after the oil.

Arrange the tomatoes on the tray with the cut side facing down and the skin side facing up toward the broiler element. This orientation is the same technique used in the Salsa Verde for the tomatillos and is equally important here. With the skin side facing the direct radiant heat of the broiler, the tomato skin will blister, char, and caramelise while the cut interior makes direct contact with the hot tray surface, concentrating the tomato's natural sugars and juices against the metal. The onion wedges and garlic cloves can be arranged in any orientation — they do not require specific positioning. Broil for 8–10 minutes. Watch carefully during the final 2–3 minutes — the difference between perfectly charred and burnt happens quickly under a hot broiler. The target for the tomatoes is a skin surface that shows deep brown to black blisters covering approximately 40–60% of the surface area — irregular, not uniform, with some areas still partially red-orange and others deeply caramelised. The onion wedges should show browning and charring at their thinnest edges and some caramelisation on their surfaces. The garlic should be golden to lightly brown with a few dark spots. Pull the tray when the tomatoes have reached this target — do not wait for uniform blackening across everything, which would indicate the vegetables are past the ideal point and would introduce bitterness into the salsa.

While the vegetables are under the broiler, prepare the ingredients that go in raw. Deseed both jalapeños by slicing them in half lengthwise and scraping out the seeds and white membrane with a spoon, then cut into smaller pieces — roughly 2–3cm chunks that the blender can process easily. Juice the lime. Gather the cilantro — stems included, for the same reason they are included in the Salsa Verde: the thin stems of fresh cilantro contain the same aromatic compounds as the leaves and blend smoothly, producing a more intensely cilantro-flavoured salsa without any textural downside in a blended preparation. The jalapeños are kept raw rather than broiled alongside the tomatoes deliberately. This is the architectural decision that gives this salsa roja its distinctive character: the tomatoes, onion, and garlic absorb the caramelised, smoky, rounded flavour of broiling — their sharpness is tamed, their sugars concentrated, their overall character deepened. The raw jalapeño and fresh cilantro added after the broil provide the fresh, sharp, slightly grassy heat and herb character that the broiled elements lack. The salsa is built on the contrast between the cooked depth of the broiled vegetables and the raw freshness of the jalapeño and cilantro — each element doing what it does best.

Transfer the contents of the tray to a blender or food processor — all the broiled vegetables and every drop of the accumulated juices on the tray. The liquid on the tray is concentrated tomato juice, rendered onion moisture, and olive oil infused with caramelised vegetable sugars — it is the most flavourful liquid in the preparation and must not be discarded. Add the raw jalapeño pieces, lime juice, and cilantro with stems. Process on high speed until fairly smooth — 20–30 seconds of continuous blending produces a salsa with a uniform, lightly textured consistency where small pieces of jalapeño and herb are still faintly detectable. For a smoother result, blend for an additional 15–20 seconds. For a chunkier result, pulse rather than blend continuously. The ideal consistency for a versatile salsa roja is smooth enough to pour from a spoon but textured enough to show the salsa's character — not a thin liquid but not a paste. Taste immediately after blending: the salsa is at its most revealing at this stage, and any significant imbalances are most easily corrected now. Season with salt — add generously in small increments, tasting after each addition, until all the flavours sharpen and come forward.

Evaluate the consistency of the blended salsa. If it is pleasantly thick and concentrated — flowing slowly from a spoon and holding a loose mound when dropped into a bowl — serve it directly. If it is thinner than desired, transfer to a small saucepan and simmer over medium heat, stirring occasionally, for 5–8 minutes until reduced to the preferred consistency. The broiling has already done the primary flavour development — the simmering step is purely a consistency adjustment rather than a further flavour development step. The salsa will not need long simmering to reduce because the broiling already drove off much of the vegetables' excess moisture. Taste and adjust the salt once more after reducing, as concentration amplifies saltiness slightly. Serve warm or at room temperature — this salsa is excellent at both temperatures, though the aromatic freshness of the cilantro is most vivid when served warm or at room temperature rather than cold from the refrigerator.