Wrap peppercorns in a clean tea towel and crush with a meat mallet or flat bottom skillet or other heavy object that will crack the peppercorns. (Don't grind the pepper it needs to be cracked).
Liberally salt both sides of the filets.
Put 1/2 the pepper on a plate in a single layer. Place filets on top of pepper and press filets into the pepper to crust. Remove filets and add remaining pepper and repeat the process on the other side of the steaks.
Heat butter and oil in a large skillet over medium heat. Once hot, increase temperature to medium-high and add the filets and sear. Cook 3 minutes for medium rare or 4 minutes for medium.
Remove from heat. Add cognac to hot pan, then carefully ignite with a long-handled match. (DO NOT ignite the cognac near the open flame of your gas stove top. Keep a lid handy in the event you need to extinguish the flame.)
Allow alcohol to burn off, about 1 minute. Return to medium-high heat, then add beef stock. Cook 1 minute.
Remove filets and place on a warm plate then cover with foil.
Continue cooking until reduced by half, about 4 minutes.
Add cream and Dijon mustard. Cook, stirring occasionally, until thick, 3–5 minutes.
Add Steaks to cognac sauce and heat for 1 minute.
Plate, spooning sauce over filet and serve hot.
Serve this classic French dish with potatoes Dauphinoise and a simple green salad.Serve french bread or croissants to soak up the yummy sauce.