In a skillet, heat 1 tbs olive oil with 1.5 tbs butter over medium-high heat. Sear steaks 2-3 minutes per side until browned and crust forms. Remove and let rest loosely tented for 5 minutes.

Prepare aromatics

In the same pan, add remaining butter if needed and the shallot and garlic. Sauté until translucent and fragrant, 1-2 minutes.

Cook mushrooms

Add the sliced mushrooms. Cook until browned and their moisture evaporates, about 5-7 minutes.

Deglaze with cognac

Carefully pour in cognac and ignite to flambé, allowing flames to burn off and alcohol to reduce, about 1-2 minutes.

Reduce with stock

Add beef stock and cook until reduced by half, about 3-4 minutes.

Finish sauce

Stir in Dijon mustard and Worcestershire sauce. Pour in heavy cream and simmer until the sauce thickens slightly, about 3-5 minutes.

Return steaks to sauce

Return steaks to the pan and spoon sauce over; cook 1-2 minutes to rewarm the beef and infuse flavor.

Finish and serve

Taste and adjust seasoning with salt and pepper if needed. Sprinkle chopped parsley on top before serving.