Preheat the oven to 400 degrees. Line two rimmed cookie sheets with parchment paper.

Core the tomatoes and slice in half. Roast for about 15-20 minutes, or until the skins wrinkle up.

Remove from the oven and set aside until the tomatoes are cool enough to handle.

Once the tomatoes are cool enough to touch, pull the skins off, and squeeze the seeds out.

Place the peeled tomatoes in the slow cooker. Add the onion, garlic, olive oil, salt, pepper, Italian seasoning, and sugar.

Cover and cook on low for 5-6 hours, or on high for 3 hours.

Once the sauce is cooked, use an immersion blender to blend it to your desired consistency. I blend until it is completely smooth.

If you aren't serving it right away, let it cool completely before refrigerating. You can also put it in freezer bags and freeze it. Makes 4 pints.