In a large skillet, heat the olive oil over medium heat. Add the Italian sausage, breaking it into smaller pieces, and cook until nicely browned.

Add the diced yellow onion and minced garlic to the skillet. Cook until the onion is soft and translucent.

Stir in the tomato paste, crushed tomatoes, Italian seasoning, salt, and water. Allow the sauce to come to a gentle simmer.

Preheat your oven to 350°F (175°C).

In a large pot, bring water to a boil and add 1/2 tablespoon of salt. Cook the spaghetti until tender but still firm to the bite. Drain the spaghetti.

Combine the drained spaghetti with the sauce in the skillet, stirring well to coat the pasta.

In a separate bowl, mix the cottage cheese, Parmesan cheese, and black pepper.

Grease an 8x11-inch casserole dish. Layer half of the spaghetti and sauce mixture, then spread the cottage cheese mixture on top, followed by the remaining spaghetti and sauce.

Sprinkle shredded mozzarella cheese over the top.

Cover the casserole dish with foil and bake for 30 minutes. Remove the foil for an additional few minutes to let the cheese bubble and brown.

Optional: Sprinkle chopped parsley over the top before serving.