Partially freeze lamb. Cut into 1/4-inch-thick strips about 3 inches long and 1 inch wide. Place in a plastic bag set in a deep bowl.
For marinade, stir together water, lemon juice, mint, mustard, oil, and garlic. Pour over lamb. Close bag. Marinate in the refrigerator for at least 2 hours or up to 24 hours, turning occasionally. Drain lamb, reserving marinade.
Meanwhile, in a small saucepan cook the zucchini, covered, in a small amount of boiling water for 2 minutes. Drain.
On five 12- to 15-inch skewers, alternately thread lamb accordion-style with onions and zucchini.
Place skewers on the unheated rack of a broiler pan; brush with marinade. Broil 3 inches from the heat for 6 to 8 minutes or to desired doneness (160 degrees F for medium doneness), turning occasionally.
Add a cherry tomato to the end of each skewer during the last 1 minute of broiling. Makes 5 servings.