Sprinkle steaks with ¼ teaspoon kosher salt. For a gas or charcoal grill, grill steaks on the rack of a covered grill directly over medium heat to desired doneness, turning once halfway through grilling. Allow 10 to 12 minutes for medium rare (145°F). Set steaks aside.
Meanwhile, for mushroom gravy, lightly coat a medium nonstick skillet with cooking spray. Add ¼ cup beef broth and garlic. Heat broth just until simmering; add mushrooms. Cook 3 minutes.
Stir cornstarch into 1 cup beef broth. Add to the mushroom mixture. Cook and stir until thickened and bubbly. Cook and stir for 2 minutes more.
To make Parsley Mashed Potatoes, place potatoes, cauliflower florets, and chopped onion in a 4- to 5-quart Dutch oven. Cover vegetables with boiling water, and simmer over low-medium heat until very tender, about 25 minutes. Drain vegetables, and transfer to a large bowl. Beat vegetables with an electric mixer on medium speed until nearly smooth. Beat in butter, ½ teaspoon kosher salt, and ground black pepper. Stir in 2 tablespoons snipped fresh parsley just before serving.
To serve, thinly slice steaks across the grain, arrange over Parsley Mashed Potatoes, and top with gravy.