This recipe multiplies well, so it's good for when you're entertaining large groups and the shells can be stuffed and assembled ahead of time.
Technique Tip: Be sure to cook the shells very "al dente" before filling them, or they will tear when you try to stuff them.
Preheat oven to 400°F. Bring a large pot of salted water to a boil for the pasta.2.
In a Dutch oven, heat the olive oil over medium heat. Add the garlic and cook until the edges are just golden, about 2 minutes. Sprinkle in the crushed red pepper. Add the tomatoes, slosh out the can with 1 cup of cooking water, and add that to the pot. Stir in the salt, and bring to a rapid simmer. Cook, uncovered, until thickened, about 20 minutes.3.
While the sauce cooks, add the shells to the boiling pasta water and cook until just al dente. Drain, and separate shells on baking sheets to avoid sticking.4.
For the filling, stir together in a bowl the ricotta, cubed mozzarella, 1 cup grated Grana Padano, the scallions, peas, parsley, and egg. Mix the shredded mozzarella and remaining grated Grana Padano in another bowl, and set aside. Stir the basil into the finished sauce.5.
To assemble, in a 10-by-15-inch or other large baking dish, spread 2 cups of the sauce. Divide the filling evenly among the shells, and arrange them in one layer in the baking dish. Top evenly with 2 cups more sauce. Sprinkle with the reserved cheese mixture. Dollop the remaining sauce evenly over the top. Tent with foil, and bake until bubbly all over, about 25 to 30 minutes. Remove foil, and bake until cheese is golden and crusty, 5 to 10 minutes more. Let sit 5 minutes before serving.