2. Bring the chicken broth to a boil, and turn down to a simmer.
3. Mash the potatoes with a potato masher or fork, or use a food mill. Stir in the sour cream. Slowly add the stock until the desired consistency is reached. Add the horseradish, salt and pepper to taste. Keep warm while you cook the steaks.
For the sirloin steaks:
1. Heat the olive oil over high heat in a skillet large enough to accomodate all of the steaks in a single layer. Season the steaks with salt and pepper and sear them in the hot skillet on both sides to the desired doneness, about 2 to 3 minutes per side for medium rare. Remove the skillet from the heat, transfer the steaks to a plate and keep warm while you make the sauce.
2. Add the shallots and garlic to the pan, return the pan to medium heat and cook for 15 seconds. Add the red wine and stir to remove any caramelized bits that are stuck to the pan. Add the beef broth and simmer until it has reduced by half, about 2 minutes.
3. Whisk in the Dijon mustard and sour cream. Spoon the sauce over the steaks and serve with mashed potatoes.