Preheat the oven to 425°F (220°C).To make the yogurt spread, combine the yogurt, 1½ tsp (7 mL) of the Dijon Mustard Rub, and the horseradish in a small bowl; set aside.Season the steak with 1 tbsp (15 mL) of the rub.Preheat the oil in a 12" (30-cm) oven-safe skillet over medium heat for 3–4 minutes. Sear the steak until it’s browned on both sides, about 4 minutes per side. Remove the steak from the skillet.Add the onion and salt, cook until softened, about 4 minutes. Place the steak on top of onions, bake until the steak reaches 145°F (63°C) for medium, about 8–10 minutes.Remove the steak and onions from the skillet. Add the broth and the remaining 1 tsp (5 mL) of the rub to the skillet; simmer 2–3 minutes.Thinly slice the steak against the grain. Spread the yogurt mixture evenly inside each roll and fill with the steak and onions. Serve with the broth for dipping.