1Heat the oven to 375°F. Bring a large pot of well-salted water to a boil and cook the shells 2 minutes less than the package says, so they stay firm enough to stuff. Drain, rinse briefly with cool water, and lay them out on an oiled sheet pan so they don't stick.
2While the water heats, warm the olive oil in a saucepan over medium heat. Cook the onion with a pinch of salt for 5 to 6 minutes until soft. Add the garlic, oregano, and red pepper flakes and stir for 30 seconds.
3Pour in the crushed tomatoes, add the sugar, and simmer gently for 15 minutes, stirring now and then, until slightly thickened. Stir in the basil and season with salt and pepper.
4In a large bowl, mix the ricotta, spinach, half the mozzarella, the parmesan, egg, and parsley. Season generously with salt and pepper. The filling should taste well-seasoned on its own.
5Spread about 1 1/2 cups of the sauce across the bottom of a 9x13 baking dish. Spoon a heaping tablespoon or two of filling into each shell and nestle them open-side up in the dish, in snug rows.
6Spoon the remaining sauce over and between the shells, then sprinkle with the remaining mozzarella. Cover tightly with foil.
7Bake covered for 25 minutes, then uncover and bake 10 to 15 minutes more until the cheese is melted, golden in spots, and the sauce is bubbling at the edges.
8Let the shells rest for 5 minutes before serving so the filling sets. Finish with a little extra basil if you like.