Heat the olive oil in a large pot over medium heat. This will help sauté the vegetables and release their flavors.
Add the finely chopped onion and sauté for about 5 minutes, or until translucent. Stir occasionally to prevent burning.
Add the minced garlic, diced carrot, and celery. Cook for an additional 5 minutes, stirring frequently. The vegetables should soften and the garlic should become fragrant.
Build the Sauce
Stir in the crushed tomatoes and tomato paste. The crushed tomatoes provide a base, while the paste adds depth and thickness.
Add the sugar, oregano, basil, salt, and black pepper. The sugar helps to balance the acidity of the tomatoes.
Bring the sauce to a simmer, then reduce the heat to low. Allow it to cook uncovered for 30 minutes, stirring occasionally. This will help the flavors meld together and the sauce to thicken.
Finish and Serve
Taste the sauce and adjust the seasoning if necessary. You can add more salt, pepper, or herbs according to your preference.
Once the sauce has thickened to your liking, remove it from heat. Serve over cooked spaghetti and garnish with chopped fresh parsley.
Notes
This sauce can be made ahead of time and stored in the refrigerator for up to 5 days or frozen for up to 3 months. Reheat gently on the stove before serving.