Heat your oven to 220°C/Fan 200°C/Gas 7. Trim the carrots and scrub them. Slice them into long chips. Pop in a bowl. Add 1 tsp olive oil. Season. Toss to mix. Spread out on a baking tray lined with baking paper. Bake for 30 mins.
Measure out 1 tsp of the cumin seeds. Add them to a dry, non-stick frying pan. Toast over a medium heat for 1-2 mins till they smell nutty and start to pop. Tip them into a pestle and mortar or spice grinder.
Measure out 1 tsp of the coriander seeds. Add to the frying pan. Toast for 1-2 mins till they smell nutty. Add to the cumin. Grind them.
Peel and grate or crush the garlic. Finely grate or pare the zest from the lime. Juice it. Mix the ground spices with the garlic, lime zest and most of the lime juice. Add the beef stir-fry strips. Stir to mix. Leave for 10 mins.
Halve the peppers. Scoop out the white bits and seeds. Finely slice them into long fingers. Peel and finely slice the onion. Separate the leaves from the lettuce. Rinse them. Pat dry with kitchen paper.
Warm 1 tsp olive oil in the frying pan. Add the beef stir-fry strips (keep the marinade). Stir-fry over a high heat for 5 mins till browned. Lift out of the pan. Pop on a plate. Keep the beef warm in the bottom of the oven.
Add the peppers and onion to the pan. Stir-fry for 3-5 mins till the veg are soft and a little charred. If it looks like they’ll stick, add a splash of water. Add the marinade. Add the beef. Stir and cook for 1-2 mins to warm everything through.
Spoon the beef and veg into a warm serving dish. Serve with the lettuce leaves for wrapping, the carrot chips and 1 tbsp yogurt each, sprinkled with the last of the ground spices.