In a large bowl, mix together the ground beef, egg, bread crumbs, salt, and steak sauce. Combine thoroughly.
Divide beef mixture into 6 equal portions, form into flat, oval shaped patties and place on wire rack over a baking sheet. Patties can also be grilled outside on a regular grill.
Place patties in oven to cook for 30 minutes, or until desired doneness, turning over once after the first 20 minutes. Remember, USDA safe minimum cooking temperature for ground beef is 165. Be sure to use your thermometer.
In a large skillet over medium heat, melt butter and add sliced onions to skillet.
Toss onions lightly to coat in melted butter. Spread the onions out evenly in the pan and let cook, stirring occasionally.
After about 20 minutes, sprinkle in salt, add the mushrooms and continue to cook for 10 more minutes, stirring occasionally.
Remove the hamburger steaks from the oven.
Top each steak with a slice of Muenster cheese and a healthy spoon of the onions and mushrooms.
Return to the oven to melt the cheese, 3-5 minutes.
Remove from oven and serve hot.
Recipe by In The Kitchen With KP at https://www.inthekitchenwithkp.com/recipes/smothered-hamburger-steak