Salt the steaks and refrigerate uncovered for about 2 hours. Remove, pat dry, and let stand at room temperature for 20 minutes.
Toss mushrooms with olive oil; roast or toast until tender, seasoning with salt and pepper halfway through.
Preheat the oven to 375°F. Mix garlic powder, paprika, black pepper, and salt. Season steaks lightly on both sides.
Heat a cast-iron skillet over medium-high heat. Sear steaks about 2 minutes per side, then transfer skillet to oven for 2 minutes. Turn off oven and let steaks finish for 6 to 8 minutes, to desired doneness.
Rest steaks. Place skillet over medium heat; add cream, salt, pepper, lemon juice, and mozzarella. Stir until combined, then stir in butter off heat.
Serve steaks with mushrooms and cream sauce.