Step 1

Place the garlic and onion in a food processor, and pulse until you get an almost paste-like consistency – everything should be pretty finely processed.

Step 2

Place a large skillet or saucepan over medium heat and add the olive oil, butter, the garlic/onion mixture, and the Italian seasoning. Cook, stirring frequently, for 10 minutes – just to allow the onions to soften, the spices to toast, and the garlic to get soft and sweet.

Step 3

Transfer this mixture to your slow cooker and add the jarred marinara sauce, and stir to combine. Allow the sauce to cook on low for at least 6, but as much as I like to leave the lid of the cooker off during the last hour-ish, as this helps concentrate/evaporate the sauce, reducing the water (not necessary though).