Stuffed Shells - Once Upon a Chef

Ingredients

• 2 tablespoons extra-virgin olive oil
• 1 medium yellow onion, chopped
• ¾ teaspoon salt
• ½ teaspoon freshly ground black pepper
• 4 large cloves garlic, minced
• ¼ teaspoon crushed red pepper flakes
• 1 (28-ounce) can crushed tomatoes
• 2 cups water
• 1 teaspoon sugar
• 1¼ cups (10 oz) whole-milk ricotta cheese
• 1 cup (4 oz) shredded fontina or whole-milk mozzarella cheese
• 1 cup (3.5 oz) grated Pecorino Romano cheese
• 2 large eggs
• 3 tablespoons chopped fresh basil
• 1½ tablespoons cornstarch
• 2 cloves garlic, minced
• 1 teaspoon dried oregano
• ½ teaspoon salt
• 25 to 30 jumbo pasta shells, from a 1-lb box (use only open, unbroken shells)
• 2 cups (8 oz) shredded fontina or whole-milk mozzarella cheese
• 1 tablespoon chopped fresh basil