Preheat oven to 500°F.
Let the bison steaks rest one hour at room temperature.
In a small bowl, knead together rosemary, salt, and butter. Rub onto the meat.
In a roasting pan, roast the meat for 10 to 15 minutes for medium-rare. Transfer to platter and keep warm.
Add the broth and cooking wine to the pan juices in the roasting pan, scraping up the brown bits. Bring to a boil, and then reduce to simmer, cooking the broth until it is reduced by half.
Stir the arrowroot mixture and add it to the gravy. Stir until dissolved. Simmer the gravy for 3 minutes, and add salt and pepper to taste. Transfer the gravy to a heated sauceboat.
Cook potatoes in medium saucepan of boiling salted water until tender, about 20 minutes. Drain well, and return potatoes to same saucepan.
Coarsely mash potatoes, and add enough chicken broth to moisten. Add a pinch of nutmeg, and season to taste with salt and pepper.
Serve under roasted filet mignon and pan gravy.