Tomato, Squash, and Red Pepper Gratin

INGREDIENTS

• 5 teaspoons olive oil, divided
• 2 cups chopped red onion
• 1 1/2 cups chopped red bell pepper
• 1 pound yellow squash, cut into 1/4-inch thick slices (about 3 1/2 cups)
• 1 tablespoon minced garlic
• 1/2 cup cooked quinoa
• 1/2 cup thinly sliced fresh basil, divided
• 1 1/2 teaspoons chopped fresh thyme
• 3/4 teaspoon salt, divided
• 1/2 teaspoon black pepper
• 1/2 cup 2% reduced-fat milk
• 3 ounces aged Gruyère cheese, shredded (about 3/4 cup)
• 3 large eggs, lightly beaten
• Cooking spray
• 1 1/2 ounces French bread baguette, torn
• 1 (12-ounce) beefsteak tomato, seeded and cut into 8 slices