Brown the sausage in a large skillet or pot over medium heat until brown and crispy on the edges. Finely dice the onion while the sausage browns. Add the onion to the skillet once the sausage is browned and continue to sauté over medium heat until soft and translucent.

Add the tomato paste, crushed tomatoes, Italian seasoning, and ½ cup water to the skillet with the sausage and stir to combine. Place a lid on the skillet and allow it to come up to a simmer. Once simmering, turn the heat down and allow it to continue to simmer while you cook the ziti. Stir the sauce occasionally as it simmers.

Begin the ziti. Bring a large pot of water with ½ Tbsp salt to a boil over high heat. Once boiling, add the ziti and let it continue to boil until tender (about 7-8 minutes). Drain the pasta in a colander, shaking out excess water.

While the pasta and sauce are cooking, prepare the cheese filling. Add the ricotta, Italian cheese blend, and some freshly cracked black pepper to a bowl and stir to combine. Preheat the oven to 350ºF / 180C / 160C fan or Gas Mark 4.

After draining the ziti, return it to the pot and add 1 cup of the red sauce. Stir to coat the cooked pasta in sauce.

Pour half of the ziti into a 9x13" baking dish. Add half of the ricotta mixture on top of the pasta in small dollops. Spoon half of the red sauce and sausage over the pasta and ricotta. Repeat these layers with the second half of the pasta, ricotta mixture, and sauce. Finally, top with 2 cups shredded mozzarella.

Cover the baking dish with foil, tenting it slightly so it doesn't touch the cheese. Transfer the covered casserole to the oven and bake for 20 minutes. Uncover and bake for another 10-15 minutes, or until the cheese is bubbly and lightly golden.