Preheat oven to 350 degrees Fahrenheit.

In a mixing bowl, combine mashed potatoes and egg.

Scoop potato mixture into eight mounds on a sprayed jelly roll pan.

Combine onion, celery, ground beef, oats, dry milk, salt, and pepper in a separate bowl.

Shape the mixture into eight meatballs.

Press each meatball halfway into a potato mound to form a cup.

Bake for 35 to 40 minutes, or until meat and potatoes are browned.

To freeze, place each potato/meatball mixture in a cupcake liner inside a muffin pan.

Bake only until the meatball browns, about 11 minutes.

Remove from oven, remove from muffin pan, cool and freeze.

Defrost in the refrigerator before baking at 350 until hot.

For gravy, melt butter in a small saucepan.

Blend in flour until smooth.

Gradually add beef broth and balsamic vinegar.

Cook and stir until smooth and thickened.

Season with thyme, pepper, and salt to taste.

Spoon gravy over potato cups and serve.