In a large pot over medium heat, add Italian sausage and cook, stirring and breaking apart with the back of a spoon, until lightly browned and releases fat. With a slotted spoon, remove meat from pan and set aside. Discard grease except for about 1 tablespoon.
Add onions, garlic, sweet peppers, and mushrooms and cook until softened.
Add browned sausage back to the pot.
Add wine and cook, stirring occasionally, for about 2 to 3 minutes or until reduced.
Add marinara sauce, crushed tomatoes, and tomato paste. Stir to combine. Bring to a boil.
Add Italian seasoning, red pepper flakes, and sugar. Season with salt and pepper to taste.
Lower heat, cover, and simmer, stirring occasionally, for 20 to 25 minutes or until meat is cooked through.
In a pot over medium heat, heat about 4 quarts water to a boil. Add ziti and cook for about 1 to 2 minutes less than the package directions. Drain and rinse under cold running water.
In a large bowl, combine ricotta cheese, Parmesan cheese, and 1 cup of the mozzarella cheese.
Add egg and gently stir without over mixing.
Add the pasta and 3/4 of the meat sauce. Gently toss together.
In a 9x13 baking or casserole dish, transfer pasta mixture and evenly spread.
Spoon the remaining 1/4 of the meat mixture on top of pasta.
Sprinkle remaining 1 cup mozzarella cheese on top of meat mixture.
Bake in a 350 F oven for about 20 to 25 minutes or until heated through and the top is golden and bubbly. Remove from oven and let stand for about 5 minutes before serving.