If you’re using leftover or jarred sauce, simply heat it up in the microwave or bring it to a simmer in a sauce pan and skip ahead. Otherwise, to make a sauce from scratch:
In a large skillet or Dutch oven, heat olive oil to medium heat. Add the carrots and onions and toss to coat with oil. Reduce heat to low, cover, and cook for 10-15 minutes, stirring occasionally, until veggies are softened.
Remove lid, add garlic, increase heat to medium high and cook until fragrant (about 1 minute). Add tomatoes, tomato paste, parsley, basil, red pepper flakes, one Tablespoon of sugar, and a generous sprinkle of salt and pepper.
Bring sauce to a gentle boil, then reduce heat to low and simmer uncovered for 30-45 minutes, stirring occasionally.
Add salt, pepper, and sugar to taste. If you prefer a smoother (less chunky) sauce, use an immersion blender to puree the sauce to desired texture.