Bring a pot of water to a boil and immerse spinach in the boiling water until dark green and softened, about 2 minutes. Drain and squeeze out excess moisture.
Cook sausage in a skillet over medium heat until browned and crumbly, about 10 minutes, stirring often. Drain excess grease. Combine spinach and sausage in a bowl. Mix cottage cheese and 8 ounces of mozzarella cheese into sausage and spinach, stirring until filling is thoroughly combined.
Use your fingers to stuff uncooked manicotti shells with stuffing. Lay stuffed manicotti into a baking dish, side by side, and pour both jars of sauce over manicotti.
Bake in the preheated oven until manicotti are tender but still slightly firm to the bite and the filling is hot, 1 hour to 1 hour and 15 minutes. Sprinkle remaining 4 ounces of mozzarella cheese over the dish, let the cheese melt, and serve.
