For the steak: If using a grill, preheat to very hot. If using a skillet, spray with cooking spray and preheat to very hot, but not smoking.
Melt 4 Tbs. of the Butter in an 8 “ skillet over medium high heat . Sprinkle with ½ teaspoons seasoned salt.
Add mushrooms and sauté for about 5 minutes until mushrooms are tender. Remove from skillet with a slotted spoon, cover with aluminum foil and set aside to keep warm.
Add remaining butter to the skillet used to cook the mushrooms. Sprinkle with ½ teaspoons of the seasoned salt.
Blot the Hash Browns with paper towels pressing out most of the liquid. Place hash browns in the skillet and press down firmly with your fingers or a wooden spoon until compressed into the skillet. Sprinkle with another ½ teaspoons seasoned salt. Cover the skillet and cook for 15 minutes over high to medium high heat until very golden brown on the bottom. Do not stir or turn with a spatula while potatoes are cooking. After 15 minutes, lift up edge of the potatoes to see if they are crispy. If not, cook another 5 minutes.
While potatoes are cooking, blot steaks with a paper towel, then rub remaining seasoned salt on both sides. Cook on very hot grill or in hot skillet for 2 ½ minutes per side for medium rare. Turn only once so steak gets a nice sear. Remove steak from heat and let rest 5-7 minutes before slicing into ½” wide slices.
To assemble: Invert a serving platter over the potatoes and flip so the Hash Browns are in a circle in the middle of the platter. Pour mushrooms on top and in the center of the potatoes. Arrange steak slices around the outside of the potatoes.