Cut the beef into 0.8 inch cubes.
Mix the meat cubes with 1 tbsp olive oil, the cumin and pepper, then marinate in the fridge for 1 hour.
Meanwhile wash the mint, shake dry and chop the leaves. Squeeze 2 tsp juice from the lemon, and mix with the yogurt, mineral water, mint, salt and pepper.
Wash and deseed the peppers, then cut into 0.8 inch pieces. Peel and finely dice the garlic and shallots. Wash and halve the tomatoes.
Thread the meat cubes onto 4 wooden skewers.
Heat 1 tbsp oil in a pan. Fry the onions and garlic for 4 minutes over a low heat. Add the peppers and cook for a further 2 minutes.
Pour in the broth, add the tomatoes, and season with salt and pepper. Bake in a preheated oven at 400°F (350°F fan, gas 6) for 10 minutes.
Meanwhile, heat the remaining oil in a griddle pan. Season the kebabs with salt, and griddle for 1 minute on each side. Remove from the pan.
Add the skewers to the peppers, and bake for another 7 minutes. Serve with the mint yogurt.