Preheat oven to 375 degrees. In a mortar, coarsely grind the coriander, mustard, juniper berries, salt and peppercorns. Transfer the spice mixture to a small bowl and stir in the olive oil and orange zest. Spread the spice paste all over the roast and set the meat, fat side up, on a rack in a large nonstick roasting pan. Pour the orange juice into the pan.
Roast the meat for 25 minutes, then lower the oven temperature to 325 degrees and roast for about 2 hours and 45 minutes, or until an instant-read thermometer inserted in the center of the roast registers 130-135 degrees for medium-rare. Transfer the roast to a carving board, cover loosely with foil and let rest 15 minutes before carving into ½-inch thick slices.