Ratatouille Slab Pie

Name: Ratatouille Slab Pie
Ingredients: 1 medium garlic head, top 1/2 inch cut off and discarded; 7 1/2 tablespoons olive oil, divided; 2 medium eggplants, unpeeled, cut into 1/2-inch pieces (about 8 1/2 cups); 3 medium-size yellow squash, cut into 1/2-inch pieces (about 6 cups); 3 medium zucchini, cut into 1/2-inch pieces (about 5 cups); 2 tablespoons kosher salt, divided, plus more to taste; 2 3/4 teaspoons black pepper, divided, plus more to taste; 1 large yellow onion, finely chopped (about 2 1/2 cups); 1 large red or yellow bell pepper (about 9 ounces), finely chopped (about 1 1/4 cups); 2 (14.5-ounce) cans petite diced tomatoes, undrained; 3 tablespoons chopped fresh basil; 2 teaspoons chopped fresh thyme; 1 teaspoon crushed red pepper; 5 batches All-Purpose Pie Dough , shaped into 2 squares and chilled; All-purpose flour, for work surface; 1 large egg, beaten