Sauté chopped onions in butter in a skillet until tender.

Transfer sautéed onions to a bowl.

Add torn bread, milk, mustard, salt, and pepper to the bowl with the onions.

Mix the ingredients well.

Add lean ground beef to the mixture and combine thoroughly.

Shape the mixture into approximately 29 small meatballs (about 1-1/4 inch size).

Place the meatballs in a greased 2-1/2 quart rectangular baking dish.

In a small saucepan, combine mushroom stems and pieces (undrained), crushed gingersnap cookies, water, brown sugar, and beef bouillon granules.

Bring the sauce to a boil over medium heat, stirring constantly, until slightly thickened.

Pour the sauce evenly over the meatballs in the baking dish.

Cover the baking dish.

Bake in a preheated 350 degree oven for 35 minutes.

Let rest for 5 minutes before serving.