Sprinkle salt and pepper on both sides of the steak and set aside at room temperature for 15 to 30 minutes.

Melt the butter and heat the oil in a large skillet set over medium-high. Pat the steak dry with a paper towel. Increase the heat to high, add the steak to the pan and sear until well browned, about 2 minutes. Turn and sear on the other side and cook until done to taste, 1 to 4 minutes longer. If your steak is thick enough, insert an instant-read thermometer in the center to test the temperature: Rare is 125 degrees; medium rare is 135 degrees; medium is 145 degrees. When the steak is done, move it to a cutting board and tent with an overturned bowl or foil to keep warm.

Add shallot to the pan and sauté until golden, 2 to 3 minutes on medium-high heat, stirring once or twice. Stir in the tomato paste, mixing well and letting it deepen in color, about 1 minute.

Turn off the heat and add the Cognac to deglaze the pan. Using a long match or stick lighter, set the Cognac on fire, standing back and taking care. Let the flames burn out, then turn the heat to medium-high and cook until the Cognac is almost evaporated, 1 to 2 minutes.

Stir in the cream, mustard, Worcestershire and fish sauce, if using, and cook until thickened, about 1 to 2 minutes. Drizzle the warm sauce over the steak — you can slice beforehand if you wish — and garnish with black pepper and herbs.