In a large skillet, heat the olive oil over medium heat. Add the chopped onion and sauté until translucent, about 5 minutes. Stir in the minced garlic and cook for an additional minute until fragrant.
Add the crushed tomatoes, diced tomatoes, tomato paste, brown sugar, salt, oregano, thyme, bay leaf, celery, and water. Stir to combine and bring the mixture to a simmer. Allow it to cook for about 30 minutes, stirring occasionally, until the sauce thickens and the flavors meld beautifully.
While the sauce is simmering, cook the sliced Italian sausage in a separate pan until browned and cooked through. Once done, drain any excess fat and set aside.
In a mixing bowl, combine the ricotta cheese with a pinch of salt and a sprinkle of oregano.
Preheat your oven to 375°F (190°C). In a 9x13 inch baking dish, spread a thin layer of the tomato sauce on the bottom. Place a layer of lasagna noodles on top, followed by half of the ricotta mixture, half of the cooked sausage, and a portion of the grated mozzarella cheese. Repeat the layers, finishing with a layer of noodles topped with the remaining sauce and mozzarella.
Cover the baking dish with aluminum foil and bake in the preheated oven for 25 minutes. After 25 minutes, remove the foil and bake for an additional 15-20 minutes, or until the cheese is bubbly and golden brown.
Once out of the oven, let the lasagna rest for about 15 minutes. This will make it easier to slice and serve.
