Instructions:
In a large bowl, mix the mashed onion (and its juice), breadcrumbs (1/3 cup), eggs, kosher salt (1 teaspoon), and black pepper (1/2 teaspoon) until combined.
Place ground beef in a bowl and mix with bread crumbs and onion mixture. I recommend using your hands for this.
Divide beef mixture evenly into 4 parts. The easiest way to do this is to press the beef mixture into the bottom of the bowl and use your fingers to “draw” lines dividing the mixture into four quadrants. Using your hands, form 4 oval patties, about 3/4 inch thick.
Heat a large skillet over high heat. Just once it’s nice and hot (after about 2 minutes), add olive oil (1 tablespoon) to the pan and mix well.
Place the patties in the hot pan and cook on each side for 2 minutes – they will be nicely browned. To get the best color, don’t move them until the two minutes are up.
Place the patty on a plate.
Add butter (2 tablespoons) to the pan, then add sliced mushrooms once melted.
Cook mushrooms for 2 minutes until golden brown.Then sprinkle flour (2 tablespoons) over the mushrooms. Stir in the mushrooms, coat each mushroom with flour and continue cooking for another minute.
Add beef broth (2 cups), stir constantly to combine and bring to a boil.
Add the steak to the sauce mixture and simmer, uncovered, for another 5 minutes or until fully cooked and the sauce thickens.