To make the marinade, toast the cumin seeds on a dry skillet until fragrant, taking care that they don’t burn. Remove, cool and crush. Place all the ingredients for the marinade in a blender or food processor and puree until smooth. Season with salt and pepper.
Transfer the marinade to a bowl, add the lamb, toss well to coat. Cover and marinate overnight in the refrigerator, turning occasionally.
To make the brochettes, blanch the bell pepper and the onions in boiling water for 2 minutes, remove, drain, and run under cold water. Remove the lamb from the marinade and place the marinade in a sauce pan and simmer for 20 minutes. Thread the meat on skewers, alternating with the pepper, onion, tomatoes and mushrooms. Brush with the reserved marinade.
Grill over a medium-hot fire until medium rare, about 15 minutes. Baste occasionally with the marinade. Cut a sample of the meat to check for doneness, and remove when a little under done.
