Preheat the oven to 375°F (190°C). Lightly grease a 2-quart baking dish with olive oil or cooking spray and spread 1/2 cup of the spaghetti sauce evenly across the bottom; set aside.
Bring a large pot of water to a rapid boil, add a generous pinch of salt, then add the 6 ounces of jumbo pasta shells. Cook for about 9 minutes (about 2 minutes less than package directions) until just tender. Drain in a colander and let shells cool slightly so they can be handled.
Meanwhile, in a medium mixing bowl combine 15 ounces ricotta cheese, 1 large egg, 1 teaspoon minced garlic, 1/4 cup freshly grated Parmesan, 1 tablespoon minced fresh parsley, 1/8 teaspoon crushed red pepper flakes (optional), 1/2 teaspoon dried oregano, 1/2 teaspoon kosher salt, and 1/2 teaspoon coarse ground black pepper. Mix until smooth and well combined.
When shells are cool enough to handle, spoon the ricotta mixture into each shell, using about 1/4 cup filling per shell. Do not overstuff; the shell should be able to close somewhat around the filling.
Place each stuffed shell seam-side up into the prepared baking dish on top of the sauce.
Pour the remaining 2 1/2 cups spaghetti sauce evenly over the stuffed shells, then sprinkle 1 cup shredded mozzarella over the top.
Bake in the preheated oven for 30 minutes, until the cheese is melted and beginning to brown.
Remove from the oven and let rest 5–10 minutes before serving.