One of best things about this recipe is that most of the ingredients you'll probably have on hand. This would not have been true, for me, a couple of months ago - because I don't use condiments such as ketchup or prepared mustard often. My sister's house, however, is well provisioned in that sense. Quite certain it's the sugar content in the ketchup that does the trick and tenderizes the meat as well.
1.3 lb of lean ground beef*
1 medium onion - rough chop
1 large celery stalk - diced fine
salt & pepper
1 TBS prepared mustard (of choice)
¼ cup ketchup
3 slices of bread - very small cubes
1 egg beaten with enough olive oil and H2O to yield @½ cup of liquid
Mix all ingredients well. Allow to sit, so bread will absorb moisture, while oven is pre-heating to 350℉. Form into oiled baking dish and bake for about 1-½ hours. Should be quite brown on top and down the sides.
*Ratio of ingredients to one another is highly flexible - use your sense of taste.