Set aside 7-8 apple slices. Roughly chop the remaining apple and place in a cast iron skillet or baking dish with the onion. Pour in olive oil and add rosemary, salt and pepper. Toss until evenly distributed.
Make small, 2-inch cuts down the length of the tenderloin. Tuck an apple slice in each of the cuts and set the tenderloin in the skillet or baking dish. Sprinkle with additional salt and pepper.
Bake 20 minutes per pound or until the internal temperature reaches 145° Fahrenheit.