Sauté together onion, garlic, green pepper, and oil in a slow cooker. Cook until onion is transparent.
Add tomatoes, sugar, parsley, oregano, basil, salt, and black pepper. Cook on Low for 2 to 3 hours. Stir frequently.
Let sauce cool. Pour sauce into quart-size freezer containers. Store in freezer.
Defrost the sauce in the refrigerator overnight or in the microwave. Transfer sauce to a saucepan over medium heat and stir in one 6-ounce can tomato paste per quart. Continue to cook, stirring frequently, until heated through.