6 ounces dry lasagna noodles
1 pound uncooked Italian sausage links
1 medium yellow onion, finely chopped
2 14.5-ounce cans diced tomatoes, or 3 1/2 cups chopped fresh tomatoes
1/2 cup chicken stock
1 tablespoon Italian seasoning
Salt and pepper to taste
15 ounces ricotta cheese
11 ounces fresh baby spinach (one large clamshell case or bag)
2 cups shredded mozzarella cheese, divided
1/2 cup grated Parmesan cheese
Boil lasagna noodles until softened but still quite al dente and a little crunchy in the middle. Drain and lay the noodles out on a clean kitchen towel.
Slice the sausage into bite-sized pieces and sauté in a heated pan with a little oil, if necessary. After a minute or two, add the onion. Continue to cook until the onion has softened and the sausage is slightly browned on all sides.
Add the tomatoes to the pan, and scrape any sticky glaze from the bottom of the pan as the juice bubbles. Add the chicken stock, Italian seasoning, and salt and pepper to taste.
While the tomato mixture simmers, mix the ricotta, 1 cup of the mozzarella, and all of the spinach in a bowl. Stir and smash until the spinach is fully incorporated into the cheese. (I also like to add a few shakes of Seasonello.)
Grease a 6-quart (or larger) slow cooker lightly with olive oil. Add a heaping cup of the sausage and tomato mixture to the bottom. Follow with one layer of noodles.
Add half of the remaining sausage and tomato mixture, followed by half of the cheese and spinach mixture, followed by another layer of noodles. Repeat.
Top with a final layer of noodles, pouring any juices remaining in the sausage pan over the top. Sprinkle the remaining 1 cup of mozzarella over top.
Cook on LOW for 3 to 4 hours. In the last 30 minutes, sprinkle the Parmesan over top.