Preheat oven to 350 degrees F. Season steaks with salt and pepper, and then coat in flour. Heat 2 Tbsp. of the oil in a large skillet set over medium-high. Brown steaks on both sides, and then place in a 9” x 13” casserole. Add the remaining oil to the skillet then add the onion, mushrooms and garlic and cook 3 to 4 minutes, until just tender. Mix in the stock, wine, chopped rosemary and mustard. Bring to a boil, and then pour over the steaks. Cover and bake until steaks are tender, about 75 minutes. Garnish each with a rosemary sprig, if desired.