Add crushed tomatoes, tomato paste, olive oil, diced onion, garlic, oregano, basil, salt, pepper, sugar (if using), and red pepper flakes (if using) to the slow cooker.

Stir everything together until evenly combined.

Cover and cook on LOW for 6–8 hours or HIGH for 3–4 hours until flavors meld.

Taste and add more salt, pepper, or sugar if needed.

For a smoother sauce, use an immersion blender directly in the slow cooker.

Serve immediately over pasta, pizza, meatballs, or let cool and store for later.