Combine everything in the slow cooker. Add crushed tomatoes, tomato paste, diced onion, minced garlic, olive oil, oregano, dried basil, bay leaves, salt, pepper, red pepper flakes, and sugar (if using) to a 6-quart slow cooker.
Stir to combine and place the lid on.
Cook on LOW for 8 hours or HIGH for 4 hours. Do not lift the lid during cooking.
Remove the bay leaves and stir well. Taste and adjust salt and pepper.
Optional — blend for smooth texture: Use an immersion blender for 10-15 seconds for a smoother sauce, or leave chunky.
Stir in fresh basil, if using, just before serving.
Serve immediately, refrigerate up to 5 days, or freeze up to 3 months.