A delicious mix of veggies and cheese make for a surprisingly hearty and easy meal, despite all the chopping. I'm not a vegetarian and I can eat this as a meal without missing meat one bit! Serve on thin spaghetti or capellini.
Melt butter in a large pot over medium-low heat; add garlic and pepper. Cook and stir mixture until garlic is fragrant, about 3 minutes. Stir carrots, celery, and onions into the butter, increase heat to medium, and continue to cook and stir until the vegetables are tender, 5 to 7 minutes.
Stir crushed tomatoes, parsley, and sugar into the vegetable mixture. Reduce heat to low and cook mixture at a simmer, stirring occasionally, for 20 minutes. Add Parmesan cheese and simmer to melt the cheese, about 5 minutes more.
Whisk cornstarch into hot water in a small bowl; add to sauce. Stir the cornstarch slurry into the sauce and cook until the sauce thickens, 1 to 2 minutes more.