In a small bowl, combine 1 teaspoon seasoned salt, the pepper and garlic salt. Rub all over the steaks.
In a large skillet, heat the olive oil over medium-high heat. Add the steaks and cook for 4 minutes on each side for medium-rare. Let stand for at least 15 minutes, then slice 1/4 inch thick.
In a small bowl, mix the mayonnaise, horseradish, mustard and remaining 1/2 teaspoon seasoned salt.
Spread a thin layer of the horseradish sauce on each toasted bread slice, then top with a piece of steak, more horseradish sauce and the parsley.