1 Tbsp. olive oil, plus more for coating the baking dish
1 lb. uncooked sweet or hot Italian sausages
1 medium yellow onion, diced
4 cloves garlic, minced
freshly ground black pepper
2 Tbsp. tomato paste
1 (28-oz) can whole peeled tomatoes
1 (14-oz) can tomato sauce
8 oz lactose-free fresh mozzarella cheese
¼ cup grated parmesan cheese
2 (15.5-oz) cans white beans, drained and rinsed
5 oz baby spinach (about 5 packed cups)
1 lb. refrigerated, shelf stable, or frozen gnocchi
fresh basil leaves, for garnish
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. (That’s what I did, and it honestly wasn’t a problem.)
Heat olive oil in a large heavy-bottomed pot or Dutch oven over medium heat until shimmering. Remove the casings from 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.
Add the onion, garlic, ½ teaspoon kosher salt, and several grinds black pepper to the pot and cook until the onion is softened, 6 to 8 minutes. Add tomato paste and cook, stirring, until well-incorporated and darkened slightly in color, 1 to 2 minutes. Add whole peeled tomatoes and their juices, breaking up the tomatoes with your hands as you add them to the pot, and tomato sauce. Taste and season with salt and pepper as needed.
Bring the sauce to a simmer and cook until thickened slightly and the flavors have developed, about 15 minutes. Meanwhile, drain mozzarella cheese if needed, then grate on the large holes of a box grater. (Pro tip: Use non-stick spray on the grater to prevent the fresh mozzarella from sticking to it too much.) Also grate the parmesan if you haven’t done so already.
Remove the pot from the heat. Add spinach to the sauce one handful at a time, stirring as you go. Stir in the white beans and gnocchi. Transfer the mixture to the prepared baking dish. Top with the mozzarella, followed by the parmesan cheese.
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.