" Heat a grill or grill pan to medium high heat. Season steak on both sides with salt and pepper. Drizzle with 2 tbsp. of the olive oil and place on the grill. Cook until nicely charred and the meat releases from the grill, about 2 minutes. Flip and cook until medium rare, about 2 minutes. Remove to a platter to rest. Meanwhile, place a large skillet over medium high heat. Add the remaining 2 tbsp. olive oil, the mushrooms, and the onion. Cook, stirring only occasionally, until the mushrooms start to brown and the onion begins to soften, about 2 minutes. Add the butter, season with salt and pepper and stir so that the vegetables brown evenly. Add the flour and stir well, so everything gets coated. Add the broth, whisking well to incorporate, and bring to a boil. Remove from heat and whisk in the sour cream. Thinly slice the steak against the grain. Divide the steak onto plates, top with the mushroom gravy and serve. "