Cut slice off top of each tomato. Scoop out center and save. Turn tomatoes upside down to drain.
Cook onion and celery in butter until soft. Chop tomato centers and add to onion mixture along with parsley, basil, garlic, salt and pepper. Cook until mixture thickens, about 15 minutes. Stir occasionally.
Add bread cubes. Place tomato shells in muffin-pan