Heat olive oil in a skillet over medium heat. Add the chopped onion, carrot, and celery. Sauté for about 5-7 minutes until softened and the onion is translucent.

Add minced garlic and sauté for an additional 1-2 minutes until fragrant. This step enhances the flavors of the sauce.

Transfer the sautéed vegetables to the crockpot.

Add the crushed tomatoes, tomato paste, and diced tomatoes. Stir well to combine.

If using, add the browned ground beef or sausage at this stage.

Sprinkle in the oregano, basil, sugar, salt, black pepper, and red pepper flakes. Stir to incorporate all the seasonings evenly.

Cover the crockpot and set it to low heat. Allow the sauce to cook for 6-8 hours. The long cooking time allows the flavors to meld beautifully.

Stir occasionally if possible, to ensure even cooking.

Taste the sauce before serving and adjust seasoning if necessary. You can add more salt, sugar, or herbs according to your preference.

Serve over cooked spaghetti or your favorite pasta.