Just cooked this up on Sunday. Rave reviews.
-4TBSP butter
-1 TBSP rosemary
-1 large onion cut into slices
-1 TBSP chopped fresh parsley
-Salt seasoning of choice to taste
-Black pepper to taste
Dry tenderloins completely. Season liberally if thick. I used seasoning salt, cracked pepper, and rosemary (light on this it goes a long way, maybe a TBSP for two loins).
Fry onions in butter slowly to caramelize. Once onions are 1/2 way done, fry loins in same pan to rare/med. rare. Pull loins off the pan constantly to slice off medallions from the thin ends. This will ensure that all medallions are not overdone! Cover with foil between.
Once thickest chunks of meat have been cooked and sliced into medallions, add onions and toss whole works in 2 TBSP parsley butter or 2 TBSP butter and one TBSP fresh chopped parsley for two loins.
Simple and incredible.
Joel