Preheat the oven to 400ºF. Wash and slice the potatoes into 1-inch pieces. Slice the kielbasa into medallions. Dice the onion into 1-inch pieces.
Add the potatoes, kielbasa, onion, and frozen green beans (no need to thaw) to a large baking sheet. Drizzle with olive oil and sprinkle the steak seasoning over top. Use your hands to toss the ingredients until everything is coated in oil and spices.
Transfer the baking sheet to the oven and roast in the oven for 20 minutes, then give the ingredients on the sheet pan a good stir. Roast for an additional 15 minutes, or until the ingredients achieve the level of browning you desire. Serve hot.