Combine rub ingredients and liberally coat goose breasts. For added flavor, wrap in plastic wrap and refrigerate for a few hours before cooking.
Heat skillet over medium heat and heat oil. Add goose, whole mushrooms and onion and brown evenly on both sides, but not past medium-rare. Remove goose from pan, keep warm and add coffee. Reduce liquid to 2 tablespoons, remove from heat and whisk in butter pieces until emulsified.
Slice goose into thin slices across the grain, arrange on plates, top with onions and drizzle sauce over.