Preheat the oven to 350°F.
Cook the elbow macaroni according to the package directions.
Drain the cooked macaroni and set it aside.
Heat a large skillet over medium heat.
Add the lean ground beef to the skillet.
Add the chopped onion, sliced mushrooms, diced celery, and diced green bell pepper.
Cook the mixture for approximately 10 minutes, or until the beef is fully browned and the vegetables are softened.
Break the beef into small crumbles while cooking.
Season the beef and vegetable mixture with salt and pepper to taste.
Drain any excess grease from the skillet, if necessary.
In a blender or food processor, combine the sugar, dried Italian seasoning, garlic powder, stewed tomatoes with their liquid, tomato paste, vegetable stock, salt, and pepper.
Blend until the sauce is smooth and evenly combined.
Stir 1/2 cup of the shredded sharp cheddar cheese into the tomato sauce.
Place the cooked macaroni in a 9x13-inch baking dish.
Add the cooked beef and vegetable mixture to the baking dish.
Pour the prepared tomato-cheddar sauce over the macaroni and beef mixture.
Stir thoroughly until the macaroni, beef, vegetables, and sauce are evenly combined.
Spread the mixture evenly in the baking dish.
Sprinkle the remaining 1 cup of shredded sharp cheddar cheese evenly over the top.
Place the baking dish in the preheated oven.
Bake for 30 minutes, or until the casserole is hot, bubbling, and the cheese is melted and lightly golden.
Remove the baking dish from the oven.
Allow the casserole to rest for 5 to 10 minutes before serving.
Serve warm.