1. Prepare grill to high heat.
2. Brush one side of each bread slice with oil; rub cut sides of garlic over oil. Spread 1/2 teaspoon mustard on each of 4 bread slices, oil side down. Top each bread slice with 1 ounce cheese, 1/3 cup greens, and 2 tomato slices. Top each with remaining 4 bread slices, oil side up.
3. Place sandwiches on grill rack coated with cooking spray; grill 2 minutes on each side or until lightly toasted and cheese melts.