Step 1

Season steaks generously on both sides with salt. Allow steaks to come to room temperature while you make the sauce.

Step 2

Stir together demi-glace (see Cook's Note), mustard, Worcestershire sauce, tomato paste, and cayenne pepper in a bowl.

Step 3

Heat oil in a skillet over very high heat, swirling carefully to evenly cover surface. When oil reaches a smoking point, transfer steaks to oil; add a few chunks of butter. Sear meat on high heat until brown on each side, 2 to 3 minutes per side; keep them on the rare side. An instant-read thermometer inserted into the center should read 125 degrees F (52 degrees C). Transfer steaks to a warm plate.

Step 4

Stir shallots into skillet; cook until softened, 2 to 3 minutes. Remove skillet from heat; pour in Cognac. Carefully ignite with a fireplace lighter. When alcohol burns off and flames go out, return skillet to high heat and bring to a boil; cook, stirring, a few minutes to reduce slightly. Add demi-glace mixture, cream, and any accumulated juices from the steak. Cook on high heat just until sauce starts to thicken, 3 to 5 minutes. Transfer steaks back to pan and reduce heat to low. Gently simmer until meat is heated through and cooked to your desired level of doneness.

Step 5

Transfer to hot plates and serve with a generous spoonful or two of sauce. Sprinkle with chives.