Preheat the oven to 350°F.
Whisk milk and potato flakes in large bowl. Add beef, 2 teaspoons salt, 1 tablespoon pepper and crushed beef bouillon. Knead until combined. I like to mix by hand, do not over mix.
Shape into eight even portions, into flat oval shapes and and transfer to parchment-lined plate. Refrigerate for 30 minutes or up to 4 hours.
Heat an oven-safe 12-inch skillet (I like to use cast iron) with deep sides over medium heat. Melt 3 tablespoons butter and cook patties until well browned, about 2-3 minutes per side. Next, place the pan into the oven for 10-15 minutes until an internal temperature reaches 165°F.
Take the steaks out of the pan and place them on a plate and cover them with foil, set aside.
Wipeout any excess grease from the pan. Over medium heat, add the remaining butter and let it melt. Add the onion and mushrooms and cook.
Stir occasionally, for about 5 minutes until the onions are softened.
Add the flour and stir it in, coating the veggies, cook stirring constantly for 2 minutes. Slowly whisk in the beef stock, making sure there are no lumps. Add the Worcestershire sauce and dijon mustard, whisk it in. Taste and add additional Worcestershire sauce if desired.
Add the garlic, pepper and tomato paste, if desired. Stir until blended. Let the gravy simmer for 3 minutes, stirring occasionally.
Add the steaks back to the pan and cover in the sauce, let simmer for 5 more minutes or until the steaks are heated through.
Garnish with a sprinkling of chopped fresh parsley. Enjoy every bite!