Saute onion and carrots for 5 minutes on low.
Add in garlic for a few minutes, being careful not to brown.
Strain tomatoes either using a food mill or colander.
This recipe is for a smooth marinara sauce. You can, however, chop the tomatoes and use entire can. If you use good tomatoes as I recommend, you will not have any waste, when using food mill.
Add in softened butter.
Season with salt pepper and herbs.
Cook on low with no lid for 2 to 4 hours, stirring frequently, or put in a crock pot on low for 6 to 8 hours.
Strain out the vegetables and herbs before serving.
If you don't have a food mill, just feel free to put all the tomatoes crushed in your hands into the pot. You will have a thicker sauce with some chunks of tomato but it's equally delicious.