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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 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 (125 degrees internal temperature 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.