Bring a large pot of water to a boil.
Fill a large bowl with ice water.
Drop tomatoes 5-6 at a time into the boiling for about 45 seconds. Transfer to the ice bath. Repeat with all tomatoes.
Peel skin off of tomatoes and squeeze out excess liquid and seeds. (The skin should very easily just peel off.)
Heat olive oil in a medium saute pan. Add onion and peppers and saute for 4 to 5 minutes until onions are translucent. Sir in garlic, salt, and pepper and cook for an additional minute.
Combine tomatoes, onion mixture, Worcestershire sauce, tomato paste, basil, bay leaves, Italian seasoning and sugar in a large slow cooker and stir to combine.
If you are making a meat sauce, stir in meat now.
Place a folded paper towel between the lid and the insert of the slow cooker and cook on low for 6 to 8 hours or on high for 3 to 4 hours.