The OED defines rasher as "1. a. A thin slice or strip of bacon, or (less commonly) of other meat, intended to be cooked by grilling, broiling, or frying; a slice of meat cooked in this way."
The etymology is given as: "Origin uncertain; probably RASH v.3 (compare sense 1 at that entry) + -ER suffix1, the original sense perhaps being �slice� or �cut�, or perhaps referring to the practice of scoring a slice of meat before grilling or frying it"
So you would not be wrong to refer to a rasher of beef, but people would look at you funny.