Prepare the Meat Sauce
Heat olive oil in a Dutch oven over medium heat. Add diced onions and cook until caramelized.
Add celery and carrots, cooking until softened.
In a separate pan, brown the ground beef and Italian sausage, breaking it into crumbles.
Add mushrooms and cook until moisture evaporates. Stir in minced garlic and thyme.
Add crushed tomatoes, tomato paste, and pasta sauce. Simmer for 45 minutes.
Season with salt, pepper, and parsley.
Prepare the Cheese Filling
In a bowl, mix ricotta cheese, cottage cheese, mascarpone, Parmesan, and Fontina.
Add a beaten egg and torn basil leaves. Stir until well combined.
Assemble the Lasagna
Preheat oven to 375°F (190°C).
Spread 1 cup of meat sauce at the bottom of a 13"x9" baking dish.
Layer in this order: noodles, ricotta mixture, mozzarella, and meat sauce. Repeat layers.
Finish with noodles, sauce, and shredded mozzarella cheese on top.
Bake the Lasagna
Cover with foil and bake for 25 minutes.
Remove foil and bake for another 25 minutes until cheese is bubbly and golden brown.
Let the lasagna rest for 15 minutes before serving.