Heat oil in a large skillet over medium-high heat. Season duck with salt, garlic powder and pepper. Add duck to pan (skin side down first, if intact). Flip duck over and brown on other side, but not past rare. Drizzle Worcestershire sauce over duck. Remove duck when rare to medium-rare and keep warm. Add cider, lemon juice and sherry to pan and reduce liquid to about 1/4 cup. Remove pan from heat and whisk in butter until emulsified. Slice duck, arrange on plates, spoon sauce over and top with onions and apple.
Notes
This sauce is also great on grilled duck halves. Just combine the sauce ingredients in a sauce pan and reduce liquid down to about 1/2 cup and drizzle sauce over cooked duck.