Thaw fish, if frozen. For marinade, in a medium bowl combine lemon juice, 1 tablespoon of the oil, the salt, thyme, and black pepper. Rinse fish; pat dry with paper towels. Cut into 1-inch cubes. Add fish to marinade; toss gently to coat. Cover and marinate at room temperature for 30 minutes. Drain fish, discarding marinade.
In a large bowl combine the remaining 1 tablespoon oil and the lemon-pepper seasoning. Add mushrooms, squash, and sweet pepper; toss to coat. Alternately thread fish, mushrooms, squash, and sweet pepper onto eight 12-inch skewers, leaving 1/4 inch between pieces. (If using wooden skewers, soak in water for 30 minutes before broiling.)
Coat the unheated rack of a broiler pan with cooking spray. Place fish skewers on the prepared rack. Broil about 4 inches from the heat for 8 to 10 minutes or until fish flakes easily when tested with a fork, turning once. Place a cherry tomato on the end of each skewer the last 2 minutes of broiling. If desired, serve the kabobs with hot cooked rice or orzo. Makes 4 servings.