I've made this recipe for stuffed bell peppers several times and love it! Since I work a lot, I use my slow cooker as often as possible, so this is a good slow cooker recipe for those who enjoy theirs as well.
http://crossfitinvasion.com/paleo-slow-cooker-stuffed-peppers-with-sausage/*Note* I tweaked the recipe a bit for my family's preference in taste. I used 12oz of tomato paste instead of 8 (2 6oz cans). We enjoy having a bit more tomato flavor, since the sausage can be a bit strong in flavor. I had to search a bit to find a tomato paste that only had tomato as the "ingredients", but was able to find some at Ralph's. Also, I only use red and orange bell peppers for the "cups" since we tend to like the sweeter tasting peppers with the sausage filling. The first time I made this, we enjoyed the taste of the filling more when it was cooked inside a green bell pepper, but didn't care for the taste of the green pepper for the "cup" as much. So, I now mince up a green bell pepper and add it to the filling. It's perfect! The green pepper seasons the filling, but allowing for the sweeter taste of the red/orange pepper "cup".

One more thing, I like to saute the onion, garlic and bell pepper before mixing the filling. It just kicks the flavor up a notch.