Cut tops off of 6-7 bell peppers. Discard seeds, leaving nice pepper cups.
Arrange tightly, cup side up in crock pot. (I diced the tops to use as stuffing.)
In a large bowl, combine the beef, egg, onion, celery, bell pepper, milk, and oats. Season with garlic, salt and pepper to your personal taste.
Combine meat mixture very well - get your hands in there! :)
Stuff meatloaf mixture into the peppers.
In a separate small bowl, combine mustard and ketchup. Whisk well and spread over the meatloaf tops.
Cook in your crockpot on low for about 6-7 hours. (High for 4-5 hours) OR Bake at 400 degrees f for 1 hour.