Heat the oil in a large frying pan over a medium-high heat. You'll know it's ready when a tiny piece of onion sizzles immediately on contact. The oil should shimmer slightly, not smoke.

Add the diced onion and cook for 2 minutes, stirring often. The onion will turn translucent and release a sweet, fragrant smell. The edges should just begin to brown — that's your cue the flavour is developing.

Stir in the minced garlic, cumin, smoked paprika, and oregano. Cook for 30 seconds, stirring constantly. The spices will become fragrant and darken slightly. Your kitchen will smell like a proper taqueria at this point.

Crumble the beef mince into the pan. Break it up with a wooden spoon as it cooks. The meat will turn from pink to brown in about 3-4 minutes. Listen for a steady sizzle — if it goes quiet, the pan is too crowded and you're steaming rather than browning.

Add the black beans and chopped tomatoes. Stir everything together and let it simmer for 2 minutes. The liquid should reduce slightly, and the mixture will look thick and glossy. Taste and adjust salt and pepper.

While the beef cooks, prepare the salad base. Shred the iceberg lettuce into thin strips, dice the tomatoes, and grate the cheese. The lettuce should be dry — pat it with kitchen paper if needed, otherwise the salad will go watery.

Assemble the salads. Divide the lettuce among four bowls, top with the hot beef mixture, diced tomatoes, and grated cheese. Add a generous dollop of soured cream on top and squeeze fresh lime juice over everything.

Serve immediately for a warm salad, or let the beef cool completely before storing. The contrast between the warm, spiced meat and the cold, crunchy lettuce is the whole point of this dish.