Prep Steaks: Let the steaks sit at room temperature for 20 minutes. Pat dry and season both sides with salt and pepper.
Cook Steaks: Heat olive oil in a skillet over medium-high heat. Sear steaks for 3–4 minutes per side (medium-rare). Transfer to a plate and cover loosely with foil to rest.
Sauté Veggies: In the same skillet, reduce heat. Add butter, then sauté onion for 2 minutes. Add garlic and mushrooms. Cook for 5–6 minutes until mushrooms are browned.
Make Sauce: Pour in beef broth, let it reduce slightly. Stir in cream and mustard. Simmer for 3–4 minutes until the sauce thickens. Season to taste.
Serve: Spoon the mushroom sauce over the rested steaks. Garnish with chopped parsley or fresh thyme.