Remove the portions of steak tenderloin from the fridge 30 minutes before cooking and let them come to room temperature. Sprinkle the steaks with 2 teaspoons each of the salt and the pepper on all sides. Heat the oven to 450 degrees F.
Warm an oven-safe skillet over medium high heat and add a tablespoon of olive oil. Heat the oil until it shimmers but doesn’t smoke.
Once the oil in the pan is hot, sear the steaks for 3 minutes on one side. To get a good sear, be sure not to move the steaks around – just place them and let them brown. Flip the steaks and add a 1 tablespoon of the butter to the pan and mix it with the fresh rosemary leaves. Sear the steaks another 3 minutes, spooning the melted butter and herbs over the steaks as they cook.
Transfer the skillet with the steaks to the heated oven, and roast for 3-6 minutes until desired doneness: 140 degrees F for medium rare, 150 degrees F for medium. I often check the temperature of the steak before I put them into the oven in order to get a good gauge of how much longer they might have to cook until desired doneness. When cooked to your liking, remove the steaks from the pan to a cutting board and let rest for 5-10 minutes.
Return the used skillet to the stove top and heat on medium low. Deglaze the pan with the wine, scraping up any brown bits, and stir in the remaining tablespoon butter until melted. Add the mushrooms, tossing with the ground mustard and remaining 1 teaspoons of salt and pepper. Cook until soft, 5-10 minutes.
To serve, pile the mushrooms and wine glaze over each steak, accompanied with a roasted vegetable of your choice. See some recommendations below.