In a medium bowl stir together the mangoes, onion, cilantro, oil, cumin, salt, and cayenne. Add tuna; stir gently to break up slightly. Cover and refrigerate up to 4 hours, if desired. To serve, spoon tuna mixture into the center of each avocado half. Serve immediately with lemon wedges. Makes 4 servings (half an avocado plus a generous 1/2 cup filling).