Prepare grill or grill pan.
Place bread on grill rack or grill pan coated with cooking spray; grill 2 minutes on each side or until lightly browned. Remove from grill. Rub cut sides of garlic over one side of each bread slice. Set aside.
Preheat oven to 350°.
Spoon ricotta onto several layers of heavy-duty paper towels; spread to 1/2-inch thickness. Cover with additional paper towels; let stand 15 minutes. Scrape into a bowl using a rubber spatula. Stir in Parmesan and next 4 ingredients (through pepper). Gently spoon about 1 tablespoon ricotta mixture into each blossom. Gently press edges of blossoms to seal in cheese.
Place stuffed blossoms in a 13 x 9-inch baking dish coated with cooking spray. Bake at 350° for 15 minutes or until thoroughly heated. Top each bread slice with 1 squash blossom; serve immediately.