Using a meat mallet, pound meat to ½ inch thickness then season with salt & pepper.
Heat 1 tablespoon olive oil over medium heat in a dutch oven. Brown steaks on each side. Remove from pan and set aside.
Add mushrooms, onions, and 1 tablespoon butter. Cook until tender, about 5 minutes.
Add steaks back to the pan. Combine remaining ingredients, except for cornstarch, and pour over steaks. Cover and bake for 2 ½-3 hours or until steaks are fork-tender.
Remove steaks from the liquid and set aside. Whisk together cornstarch and 2 tablespoons water until smooth. Bring cooking liquid to a boil and add cornstarch mixture slowly while whisking continuously, until desired thickness.