Pull steak out of the refrigerator and bring to room temperature. Preheat the oven to 450F.
In a medium frying pan, add 2 tablespoons olive oil, mushrooms and onion, bring to medium/high heat. Stirring constantly while sauteing until soft and caramelized, about 8-10 minutes.
Coat steak in remaining olive oil and season generously with freshly ground pepper and salt on both sides.
Bring the oven-safe cast iron pan to high heat and sear steak on one side, about 4 minutes. Flip the steak and place the pan in the preheated oven. Cook for 10-12 minutes until cooked through or desired doneness.