Instructions

1. Arrange a rack in the middle of the oven and heat the oven to 425°F. Lightly coat a lasagna pan (a 9x13-inch pan that’s at least 3 inches deep) or 3-quart baking dish with olive oil. Alternatively, you can use a 9x13-inch baking dish that's 2 inches deep, but it will be very full.
2. Heat 1 tablespoon olive oil in a large heavy-bottomed pot or Dutch oven over medium heat until shimmering. Remove the casings from 1 pound uncooked Italian sausages, add to the pan, and cook, breaking up the meat into smaller pieces with a wooden spoon, until browned, beginning to crisp, and cooked through, 6 to 8 minutes. Meanwhile, dice 1 medium yellow onion and mince 4 garlic cloves.
3. Add the onion, garlic, 1/2 teaspoon kosher salt, and several grinds black pepper to the pot and cook until the onion is softened, 6 to 8 minutes. Add 2 tablespoons tomato paste and cook, stirring, until well-incorporated and darkened slightly in color, 1 to 2 minutes. Add 1 (28-ounce) can whole peeled tomatoes and their juices, breaking up the tomatoes with your hands as you add them to the pot, and 1 (15-ounce) can tomato sauce. Taste and season with salt and pepper as needed.
4. Bring the sauce to a simmer and cook until thickened slightly and the flavors have developed, about 15 minutes. Meanwhile, drain 8 ounces fresh mozzarella cheese if needed, then grate on the large holes of a box grater. Grate until you have 1/4 cup Parmesan cheese. Drain and rinse 2 cans of white beans.
5. Remove the pot from the heat. Add 5 ounces baby spinach (5 packed cups) to the sauce one handful at a time, stirring as you go. Stir in the the white beans and 1 pound gnocchi. Transfer the mixture to the prepared baking dish. Top with the mozzarella, followed by the Parmesan cheese.
6. Bake until the casserole is bubbling and the cheese is browned, 30 to 35 minutes. Let cool for at least 10 minutes. Top with fresh basil leaves just before serving.