12 dried lasagna noodles (about 10 ounces), uncooked
2 teaspoons olive oil
1 large onion, chopped
4 garlic cloves, finely chopped
1 1/2 teaspoons dried Italian seasoning
1/4 teaspoon red-pepper flakes
1 pound sweet Italian sausage, casings removed
2 cans (28 ounces each) whole peeled tomatoes in puree
Coarse salt and ground pepper
6 cups shredded shredded mozzarella cheese (about 1 1/2 pounds)
3/4 cup shredded Parmesan cheese (about 3 ounces)
Fill a 9-by-13-inch baking dish two-thirds of the way with hot tap water. Add noodles, and arrange them in alternating directions to prevent sticking. Let soak while preparing sauce.
Make sauce: Heat oil in a large saucepan over medium. Add onion, garlic, Italian seasoning, and red-pepper flakes; cook, stirring occasionally, until lightly browned, about 5 minutes. Add sausage, and cook, stirring and breaking it up, until just cooked through, about 5 minutes. Add tomatoes and their puree, breaking up tomatoes with a spoon (or use emulsion blender); bring to a boil. Reduce to a rapid simmer, and cook, stirring occasionally, until thickened, about 20 minutes. (You should have about 8 cups of sauce.) Season with salt and pepper.
Preheat oven to 350 degrees. Drain noodles, transfer to a baking sheet; dry dish with paper towels. Spoon 1 cup sauce in bottom of dish. Fit 4 noodles in dish (crosswise if using short noodles, lengthwise if using long), overlapping slightly. Cover with 2 cups sauce, 2 cups mozzarella, and 1/4 cup Parmesan. Repeat with another layer, then top with remaining noodles, 3 cups sauce, and the rest of mozzarella and Parmesan. Cover dish tightly with aluminum foil, and bake 1 hour.
Remove foil, and continue baking until bubbly and browned, about 30 minutes. Remove from oven; let cool 20 minutes. Cut and serve.