In a small bowl, combine the 120g of mayonnaise, 30g of harissa paste, the finely grated garlic clove, and 1 tbsp of lime juice. Stir until completely smooth and uniformly coloured — no visible harissa streaks remaining. Taste: it should be simultaneously smoky from the harissa, creamy from the mayonnaise, garlicky, and bright from the lime. The harissa's specific fermented, smoke-spiced character is the defining flavour of this sauce — its North African-inspired complexity is what distinguishes this burger from a generic hot-sauce mayo. Add additional lime juice if the brightness is insufficient; add a pinch of salt only if the overall flavour tastes flat. Refrigerate until assembly — the cold temperature is part of the sauce's function in the burger, providing a cool, creamy contrast against the warm halloumi and sweet peppers.

Heat a large cast iron or heavy stainless steel pan over medium-high heat until genuinely hot — the oil should shimmer immediately on contact. Add the 1 tbsp of neutral oil. Add the sliced red and yellow bell peppers and white onion simultaneously. Season lightly with black pepper and add salt only after tasting — the peppers' natural sweetness is sufficient seasoning in most cases. Cook for 8–10 minutes, tossing frequently but allowing brief stationary contact with the hot surface between each toss — the moments of direct contact are what produce the caramelised edges and scattered charred spots that give the peppers their specific sweet, slightly smoky depth. The peppers and onion should be noticeably softened and golden at the edges, still retaining some structure rather than collapsing completely. Transfer to a bowl and keep warm while the halloumi is cooked.

Pat each halloumi slab completely dry on both surfaces with paper towels — the surface moisture that halloumi carries from its brine prevents immediate Maillard contact and produces a pale, steam-softened exterior rather than the deeply golden, slightly crisped surface the burger requires. For a softer, more yielding interior alongside the developed crust: place the dried halloumi slabs on a microwave-safe plate and microwave for 10–20 seconds before cooking. This gentle pre-warming heats the centre so the pan only needs to develop the exterior crust — producing a burger-friendly texture where the inside remains soft and tender rather than the firmer, slightly rubbery result of cold halloumi cooked entirely from the pan's heat. Heat a grill, ridged grill pan, cast iron skillet, or heavy stainless steel pan over medium-high heat. Add the 1 tbsp of neutral oil. Place the prepared halloumi slabs in the pan. Cook for 2–3 minutes undisturbed — the sustained contact developing the deeply golden crust before the flip. Flip each slab and cook the second side for 2–3 minutes until equally golden. The correctly grilled halloumi should have a clearly caramelised, slightly crisped exterior surface and a soft, yielding, warm interior.

Slice each brioche bun in half. Spread the cut surfaces of each half with a thin layer of the 30g of unsalted butter. Toast cut-side down in the same pan used for the halloumi over medium heat for 1–2 minutes until the cut surfaces are golden-brown and slightly crisped. If using our Brioche Burger Buns, their enriched, slightly sweet crumb toasts to a specifically golden, butter-crisped surface that provides both structure and flavour that commercial buns cannot match — the butter's milk solids browning against the hot surface adding extra depth. Remove from the pan immediately when golden — over-toasted brioche becomes dry and brittle.

Spread a generous layer of the cold harissa mayo onto the bottom bun. Add a small handful of fresh rocket — the arugula's peppery, slightly bitter freshness providing the specific green element that cuts through the rich halloumi and sweet peppers. Add a generous portion of the warm roasted peppers and onions over the rocket. Add the optional pickled red onions or cucumber pickles at this stage if using — their sharp acidity amplifying the lime in the harissa mayo and providing additional contrast against the sweet peppers. Place the grilled halloumi slab directly on top of the peppers. Spread another generous layer of harissa mayo onto the inside of the top bun. Crown the burger. Press gently to compress the layers — enough that the burger can be bitten into without components sliding, but not so firmly that the soft halloumi is squeezed out. Serve immediately while the halloumi is still warm and yielding, the peppers remain sweet and slightly smoky, and the contrast between warm filling and cold spiced mayo is at its best.