Season 4 filet mignon pieces with a little olive oil, salt and pepper.
Heat cast iron skillet.
Add butter and oil on medium.
Once melted, add filets and sear a couple mins.
Flip and sear second side. Remove to a plate and cover.
Deglaze slowly and carefully with cognac. (Tip: add small amount so it doesn't flame up)
Add broth and cream, stir to combine sauce. Add mustard if using.
Slice steaks thinly and return to the sauce just a few minutes until steak is cooked to your liking. (flip pieces)
Top with chopped chives, salt/pepper if needed.
