Thaw tilapia fillets, if frozen. Preheat broiler. Sprinkle fish with the salt and cayenne pepper. Arrange fish on a greased unheated rack of a foil-lined broiler pan. Broil 4 inches from the heat for 4 to 6 minutes per 1/2-inch thickness of fish or until fish flakes when tested with a fork. Break fish into bite-size pieces. Cover and keep warm.
Arrange spinach on a serving platter; set aside. In a large skillet heat oil over medium heat. Cook mushrooms, leek, and garlic in hot oil about 4 minutes or until tender. Add tomato and cook about 1 minute more or until softened. Add vinegar and heat through. Add mixture to spinach and toss to coat and lightly wilt. Top with fish and sprinkle with snipped fresh thyme.