In a 6 quart slow cooker, combine the onion, garlic, tomato paste, olive oil, balsamic, sugar, basil, oregano, thyme, salt, and crushed red pepper flake, stirring to combine. Top with the bay leaf.
Split the tomatoes in half crosswise. Squeeze out the seeds (it’s ok if some are missed) into a bowl or fine strainer with a bowl underneath. Cut any large halves in half again (smaller tomatoes don’t need cut again). Place in the slow cooker on top of the onion mixture and do NOT stir. Cover and cook on high for 3 hours.
Stir the content of the slow cooker. Find and remove the bay leaf. Use an immersion blender to puree the mixture until smooth (lumps could be tomatoes with skin, so smooth is best).
Cover and cook again for 5 hours on high or until thickened. Taste test and add more salt as necessary.
Refrigerate for up to 5 days or freeze for up to 6 months. Makes approximately 9 cups, or just over 2 quarts.
Recipe by Alison's Allspice at https://www.alisonsallspice.com/no-peel-slow-cooker-marinara/