Slow Cooker Tomato Sauce

1 teaspoon olive oil
2 onions, chopped
5 garlic cloves, minced
2 carrots, finely chopped
3 celery ribs, chopped
1 green pepper, chopped
1 red pepper, chopped
2 (28 ounce) cans tomatoes, crushed
1 (15 ounce) can tomato sauce
4 tablespoons tomato paste
1 (10 ounce) can tomato soup
2 teaspoons dried sweet basil leaves
1 tablespoon parsley
1 teaspoon dried oregano leaves
1/2 teaspoon thyme
1/2 teaspoon salt
1/2 teaspoon pepper
1/4 teaspoon cayenne
1 dash red pepper flakes

Heat oil in skillet; add onions,garlic, celery peppers and carrots.
Saute on high until transparent.
Transfer to slow cooker.
Add remaining ingredients.
Stir to blend together.
Cook on low for 7 to 9 hours.
About an hour before serving time, check and thin with water if necessary.
I love a thick sauce.
Last time I made this I have a large slow cooker; I added a 3rd can of chopped tomatoes.
Set the Slow cooker on Low and let is cook overnight.