Heat a large frying pan over medium high heat. Sprinkle pork on all sides with salt, pepper and garlic powder; rub into meat. Sear the meat on all sides until lightly brown. Transfer to a roasting pan.
Place onion slices over meat and in the roasting pan. Fill the pan 2/3 full of water. Cover and place in preheated oven for 3 hours. Add the potatoes and carrots; cover and cook 45 minutes. Add the mushrooms and cook another 15 minutes. Remove and let stand at least 10 minutes before serving.