In a large bowl mix together the milk and potato flakes. Using your hands’ mix in the ground beef, ½ teaspoon salt, and ½ teaspoon pepper; form meat mixture into 2 patties about a ½ inch thick.
Melt 1 tablespoon of butter in a nonstick skillet over medium heat. Brown patties on both sides and set aside on a plate.
Add a tablespoon of butter to the skillet and the onions; cook until onions start to brown. Add the mushrooms and a ¾ teaspoon of salt and cook until mushrooms have given up their moisture and are tender. Stir in the flour and tomato paste and cook for about 2 minutes. Slowly mix in the broth and sherry. Return the patties to the skillet, cover, bring to a simmer, and cook covered for 5 minutes. Flip patties and continue cooking covered until patties are done. (160F)
Remove steaks to a platter and lightly cover with aluminum foil to keep warm. Continue cooking the sauce until slightly thickened – salt and pepper to taste and pour the sauce over steaks.