2 6-ounce beef fillets, cut from the tenderloin (filet mignon), preferably not too lean
Salt and pepper
1 tablespoon extra virgin olive oil
2 tablespoons butter
1 tablespoon minced shallot or onion
1 teaspoon Dijon mustard
1 teaspoon Worcestershire sauce, or to taste
1/2 cup heavy cream or half-and-half
Lemon juice to taste, optional
Chopped fresh chives or parsley leaves for garnish
Flatten fillets a bit with the palm of your hand, the back of a skillet or a small mallet; they should be about 1 inch thick. Sprinkle with salt and a lot of pepper. In small skillet, preferably one just large enough to hold fillets, combine oil and tablespoon of butter over medium-high heat. When the butter foam melts, sear steaks on both sides, just until browned, no more than 2 minutes a side. Remove to platter.
Wipe pan clean with towel; add remaining butter over medium heat, with shallot or onion. Cook, stirring occasionally, until tender, about 2 minutes. Stir in mustard, Worcestershire and cream. Add some salt and a fair amount of pepper. Stir once or twice, then taste and adjust seasoning.
Keeping mixture at a steady simmer, return meat and accumulated juices to pan. Cook, turning two or three times, until meat is done to your liking, just 1 or 2 minutes a side for medium-rare. Remove to a plate, and add lemon juice, if using, salt and pepper to the sauce as needed. Spoon sauce over meat, garnish with chives or parsley, and serve.