Beef Cordon Bleu Casserole

Ingredients
1-1/2 lbs ground round beef
1 large yellow onion, diced
1 clove garlic, diced
3 large russet potatoes, peeled and chopped
1-1/2 cups milk
1/4 lb butter, divided
2 tbsp white vinegar
1 cup yellow cheese, shredded
salt and pepper to taste

In a large skillet saute beef, onion, garlic and vinegar (this is
the secret) until browned; drain.
Meanwhile boil potatoes in salted water just covering them until
done, about ten minutes; drain and add milk and half of the butter.
Salt and pepper to taste.
Whip or mash until smooth.
Grease a casserole dish and spread meat/onion mixture.
Pour potatoes on top and dab with remaining butter.
Top with shredded cheese.
Bake at 375 degrees for fifteen minutes.