Preheat oven to 350ºF. Season steaks with garlic powder, salt, and pepper. Place on a baking sheet. Cover very well and bake for 1 ½ hours.
Meanwhile, heat oil in a saute pan. Add onion and garlic and saute for 3-4 minutes. Add mushrooms and saute, stirring occasionally, for 2-3 additional minutes. Add teriyaki sauce, vinegar, and sugar, and saute for an additional 2-3 minutes. Season with salt and pepper.
Lower oven temperature to 275ºF. Pour mushroom mixture over steaks and bake uncovered for an additional 1 hour.
Recipe by Between Carpools at https://betweencarpools.com/cooking-oven-teriyaki-steaks-in-the-oven/