Separate the sirloin into 4 slices, remove excess pieces and place in a bowl. - Add the mustard, a few grains of crushed pepper and mixed herbs. Work the mixture with your hands and let it rest in the refrigerator for at least 1 hour. - Sprinkle the salt flakes and butter on a pan. As soon as the pan is hot, cook the buffalo steaks for 2-3 minutes per side, being careful that you do not pierce the meat while turning it. In the meantime, prepare a bed of herbs, where the meat will rest the meat and also acquire flavor. - Once the meat is cooked, place it on the bed of herbs and cover with more herbs. Prepare the gravy by adding 1/2 cup of water to the pan and simmering it until the sauce is thick. - After 5 minutes cut and serve the meat, garnished with herbs, some salt flakes and gravy