Heat the oil in a large saucepan over medium heat until shimmering. Add the onion, salt, and pepper.
ViewMore
Cook, stirring occasionally, until softened and lightly browned, about 10 minutes. Stir in the garlic and red pepper flakes and cook until fragrant, about 30 seconds (don’t brown the garlic).
ViewMore
Stir in the crushed tomatoes, water, and sugar and bring to a gentle boil. Reduce the heat to medium-low and cook, uncovered, until flavors have melded, about 5 minutes. (The cooled sauce can be refrigerated for up to 3 days.)
ViewMore
Combine the ricotta cheese, shredded fontina (or whole-milk mozzarella) cheese, Pecorino Romano cheese, eggs, basil, cornstarch, garlic, oregano, and salt in a bowl.
ViewMore
Mix until thoroughly combined.
ViewMore
Transfer the filling to pastry bag or large zipper-lock bag. (If using a zipper-lock bag, be sure the corners are square; the rounded-corner bags are difficult to use.)
ViewMore
Adjust the oven rack to middle position and preheat the oven to 400°F. Place the shells open side up on the counter. Cut ½-inch off of the tip or corner of the bag.
ViewMore
Pipe the filling into shells until each is full.
ViewMore
Spread 1 cup of the sauce over the bottom of 9×13-inch baking dish. Transfer the shells, open side up, to the prepared dish.
ViewMore
Pour the remaining sauce evenly over the shells to completely cover.
ViewMore
Cover the dish tightly with aluminum foil. Bake until the shells are tender and the sauce is boiling rapidly, about 45 minutes. Remove the dish from the oven and discard the foil.
ViewMore
Sprinkle the fontina (or mozzarella) over top. Return to the oven and bake, uncovered, until the cheese is lightly browned, about 15 minutes.
ViewMore
Let the dish cool, uncovered, for 15 to 20 minutes (this rest is essential to fully cook the pasta).