Summer Gratin

Bahan-bahan (6 porsi)

• 5 tablespoons olive oil, (divided)
• 2 medium onions, (thinly sliced)
• 1 medium garlic clove, (minced)
• 1¼ pounds small or medium, ripe tomatoes ((cored and cut into 1/4-inch slices))
• ¾ pound (2 small) zucchini, (cut into 1/4-inch slices)
• ¾ pound (2 small) yellow summer squash, (cut into 1/4-inch slices)
• ¼ cup chopped fresh thyme, (divided)
• 1 teaspoon salt, (divided)
• 1¼ cups grated Parmesan cheese, (divided)
• freshly ground black pepper, (to taste)