Combine vinegar, mustard, salt, rosemary, and pepper in a large resealable plastic bag or container for marinating. Add onion and bison, and toss to coat. Let marinate in refrigerator for at least 30 minutes, preferably several hours.
Heat grill to medium-high. Grill steak to preferred doneness (it is recommended to not cook the bison beyond medium, as it is lean and will become dry). For a side dish, cook the onions on low heat in a skillet until they are soft and caramelized.
Recipe courtesy of Three Suns Ranch.