Prepare the lamb by cutting it into 1” cubes.
Find a large food storage container with a lid, and transfer the lamb into it.
Cut up the onion and pepper into large pieces suitable for the skewers.
Put everything together in the container and mix thoroughly
Leave the container in the refrigerator covered overnight.
The next day, bring the mixture to room temperature.
Begin assembling the skewers by placing lamb, followed by onion and tomato, followed by lamb, pepper, and mushroom. This assembly not only looks good, but the onion piece holds the tomato secure, and the pepper piece holds the mushroom.
Place on a high-heat grill (oil the grates first).
Turn every two minutes for about 10 minutes if you want medium rare.