Season the beef with salt and pepper, to taste.
Heat 2 tablespoons the oil in a large cast iron skillet over medium-high heat until very hot but not smoking. Add the meat and on all sides.
Add the mushrooms and to a bowl and stir in the remaining 2 tablespoons of oil. Season with salt and pepper, to taste, and toss. Add to the skillet around the roast.
Put the skillet in the oven and roast until an instant-read thermometer registers 130 degrees F, about 40 minutes. Remove the from the oven to a cutting board and let rest for 10 minutes. Remove the onions and mushrooms from the skillet.
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