Step 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.

Step 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.

Step 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.

Step 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.

Step 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.

Step 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.