Spray a 7-quart slow cooker with cooking spray.
Combine ground beef, onion, bell pepper, breadcrumbs, oats, egg, tomato sauce, salt, pepper, and garlic.
Make foil handles. See tips below.
Shape into one large loaf and place it in the center of the crock-pot on top of the foil handles.
Add ½ of the sauce before baking.
Cook on high 3 12/ hours or low 6 to 7 hours.
The meatloaf is done once it reaches 170 F degrees but take it out of the crock-pot or turn it off once it reaches 165 degrees. This will keep it from drying out. It will finish cooking.
Combine all the ingredients for the sauce in a saucepan and bring to a boil.
After you bring the sauce to a boil, cut the heat and simmer for 5 minutes.
If using your crock-pot, add half of the sauce on top of the meatloaf in the slow cooker. Add the other half of the sauce the last 30 minutes it is in the slow cooker.