Place a liner in a slow cooker.
Pour tomato sauce and tomatoes in a slow cooker.
In a small food processor, blend red peppers to desired texture.
Pour peppers into the sauce.
Add seasoning pouch and pesto or basil to the sauce.
Cook on low for 8-10 hours. Stirring a couple of times to keep edges from burning.
About 20-30 minutes before eating, stir in frozen spinach.
Serve as you would any marinara sauce.