Cut up the pork tenderloin into slices about one inch thick. Sprinkle garlic powder and black pepper over all the slices of pork. Set the pork aside until later.
In a large skillet, sauté onions and bacon together over medium heat. Cook until bacon has cooked thoroughly. Remove from pan and set aside on a separate plate or bowl.
Add the butter if there is no bacon grease left in the pan. Increase the heat to medium to high and sear all the sliced pork tenderloin until brown on both sides.
Pour in chicken stock to skillet. Top the pork with onion bacon mixture and sliced mushrooms. Cover the skillet and reduce the temperature to low to medium heat. Cook for about 20-30 minutes.
While the pork braises, boil the water and cook homemade apple pie pierogis.
Plate your cooked pierogis with sliced pork and sautéed bacon mushroom onion mixture on top.