Heat the olive oil in a frying pan over medium heat. Season pork loins with salt, pepper and garlic powder. Cook pork in the oil, turning until browned on all sides, 5 to 6 minutes. Remove from frying pan and set aside. In the same frying pan, cook onion and mushrooms until they brown lightly and begin to soften; 3-5 minutes.
Pour wine into slow cooker; stir in the onion and mushrooms. Place browned pork on top of the vegetables.
Combine mushroom soup, Dijon mustard and sour cream in a bowl. Spread mixture over pork. Cover slow cooker; cook on Low until pork is tender and liquids have formed a sauce; 7 to 8 hours.
I would not cook on High as this will dry the meat out.