Preheat oven to 200°F.
Season both sides of steaks with kosher salt.
Coarsely crush peppercorns in a sealed plastic bag with a meat pounder rolling-pin, then press pepper evenly onto both sides of steaks.
Heat a heavy skillet over moderately high heat until hot, add oil, and sauté steaks in 2 batches, turning over once, about 6 minutes for medium-rare.
Transfer steaks as cooked to a heatproof platter and keep warm in oven while making sauce.
Pour excess fat from skillet leaving the browned bits on the bottom of pan. Add shallots and half of butter (2 tablespoons) to skillet and cook over moderately low heat, stirring and scraping up brown bits, until shallots are well-browned, 3 to 5 minutes.
Remove pan from heat and add the cognac or brandy. Return pan to heat and boil, stirring, until liquid is reduced to a glaze, 2 to 3 minutes. Add cream and any meat juices accumulated on platter and boil sauce, stirring occasionally, until reduced by half, 3 to 5 minutes. Add remaining butter and Dijon mustard and cook over low heat until butter is incorporated. Serve sauce with steaks.