Lean sirloin roast should be cooked rare so it stays tender; the mushroom jus helps keep the meat moist. The roast must cure overnight, which will also keep it juicy. Rewarm the sliced leftovers in the jus and serve them on kaiser rolls.
In a large saucepan, combine the water, garlic, salt, sugar, allspice and bay leaves. Bring to a boil and cook for 2 minutes, then let the brine cool to room temperature. In a very large bowl, cover the buffalo roast with the brine and refrigerate overnight.
Preheat the oven to 325°. Pat the buffalo roast dry with paper towels. Season with salt and pepper. In a large skillet, heat the oil until almost smoking. Add the roast, widest side down, and brown over high heat, about 4 minutes. Continue cooking, turning, until browned all over, about 10 more minutes. Transfer to a large roasting pan and cook for about 1 1/2 hours, or until an instant-read thermometer registers 120° for very rare. Let the roast rest for 20 minutes.
Cut the roast in half across the grain. Thinly slice each half across the grain. Arrange the slices on plates and serve with the Mushroom Jus.