This is something I've invented to go along with a steak barbecue, to use up some extra peppers that were going to go bad. I would suggest that you use red and yellow peppers, as green ones can be a little too bitter.
Tasty Pepper Filling:
Put the bread in a food processor and chop it up into tiny pieces. Chop the garlic, celery, and onions. Grate the cheese, in any ratio you choose (though I would caution that parmesan can be overpowering). Combine all of the ingredients in a bowl and knead together until you've got something reasonably uniform. Cut the top off of the peppers, scoop out the insides, and put a little bit of salt at the bottom (draws out some pepper-juices, makes things tasty). Scoop 'em full of the filling and top with a little cheese. Put in a casserole dish with a little bit of olive oil and bake at around 400 Fahrenheit for about 45 minutes.