Coarsely chop onion. Melt 1 teaspoon (5 mL) butter in a large frying pan over medium heat. Add onion. Stir often until softened, 4 to 5 minutes. Meanwhile, slice pork in half lengthwise. Place cut-side down on board and flatten slightly with your palm. Cut each half lengthwise into 5 to 6 long thin slices. Slice crosswise to form bite-size strips. Remove onion to a large plate.
Melt 1 teaspoon (5 mL) butter in pan. Add pork. Stir often until golden-tinged from 5 to 6 minutes. Remove to plate with onions. Increase heat to medium-high. Melt remaining butter in pan. Add mushrooms. Sprinkle with salt. Stir often until cooked through, 3 to 4 minutes. Meanwhile, stir sour cream with Dijon and Worcestershire. Once mushrooms are cooked, reduce heat to medium. Return onions and pork to pan. Add sour cream mixture. Stir often until warm. Remove from heat. Sprinkle with green onions. Great over egg noodles or rice.