Make the sauce: Pour the olive oil into a large pot, add the pancetta and cook over medium-high heat until golden brown, 4 to 5 minutes. Add the onion, season generously with salt and cook until soft but not browned, 6 to 7 minutes, stirring frequently. Add the garlic and red pepper flakes and cook 2 to 3 minutes, stirring.

Pass the tomatoes through a food mill (or puree in a food processor) and add to the pot. Fill one of the empty tomato cans with about 2 1/2 cups water, add to the pot and season generously with salt. Simmer 2 to 3 hours, stirring occasionally. Taste frequently and season with salt as the sauce simmers-tomatoes take a lot of salt.

Make the lasagna: Preheat the oven to 375. Bring a large pot of generously salted water to a boil (the water should taste like the ocean). Add the lasagna noodles and cook until al dente, about 10 minutes. Fill a bowl with salted ice water. Remove the pasta from the boiling water and transfer to the ice water to stop the cooking.

Mix the ricotta, pecorino, 1/4 cup parmesan, 1 1/2 cups mozzarella, eggs, thyme and 2 teaspoons salt in a bowl. Drain the pasta and place on a clean towel in a single layer.

Coat the bottom of a 9-by-13-inch baking dish with a ladle or two of sauce. Spread 3 heaping tablespoonfuls of the cheese filling down the center of each lasagna sheet; roll up the sheet (but not too tightly or the filling might ooze out). Place seam-side down in the baking dish, leaving a little room between each roll. Cover with 3 to 4 cups sauce and sprinkle with the remaining 1/4 cup parmesan and 1 1/2 cups mozzarella.

Bake, uncovered, until the lasagna bubbles around the edges and the cheese melts, about 1 hour. Let rest 10 minutes, then serve with extra sauce.

Photograph by Ngoc Minh Ngo