Preheat the oven to 375℉ (190℃). Grease a 2-quart baking dish very lightly with olive oil, spread ½ cup spaghetti sauce on the bottom of the dish, and set to the side.
Bring a large pot of water to a rapid boil to cook the shells. Once the water is boiling, heavily salt the water, and add 6 ounces jumbo pasta shells. Cook for 9 minutes, or 2 minutes less than the package directions state. Drain the pasta, but do NOT rinse, and allow the shells to cool slightly.
While the pasta is cooking, prepare the stuffing for the shells, by mixing 15 ounces ricotta cheese, 1 large egg, 1 teaspoon garlic minced,¼ cup parmesan cheese, 1 tablespoon fresh parsley, ⅛ teaspoon crushed red pepper flakes, ½ teaspoon dried oregano, ½ teaspoon kosher salt, and ½ teaspoon coarse ground black pepper in a medium mixing bowl. Set that aside to stuff the shells.
Once the shells are cool enough to handle, stuff each shell with the ricotta mixture, about ¼ cup filling per shell. Remember to not overstuff the shells, so that the shells can somewhat close around the ricotta filling. Place each stuffed shell into the prepared baking dish.
Top the shells with what is left from the 2½ cups spaghetti sauce and then cover with the 1 cup shredded mozzarella cheese.
Bake for 30 minutes, or until cheese is melted and browned.
Let sit for 5-10 minutes prior to serving.
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Notes
Adding Spinach: Stir in ½ to 1 cup of well drained, chopped spinach to the ricotta mixture. Adding Meat: Mix in ½ pound cooked ground beef, turkey, or Italian sausage to the filling OR sauce.Leftovers: Store in an airtight container in the refrigerator for up to 4 days and reheat as desired. To Prep and Freeze Ahead: Assemble the stuffed shells in a freezer-safe or disposable baking dish, cover tightly with two layers of foil, and freeze for up to 3 months. When ready to bake, remove one layer of foil and bake at 350°F for 1 hour. Uncover and bake 10-15 more minutes, until hot and bubbly.