When I think prime rib, I think fatty, unctuous, melt-in-the-mouth tender, celebratory, not-every-day-if-I-don't-want-to-die-young-and-happy, and other such terms. Lean is not a word that comes to mind. Yet when we're working with bison, that's its primary distinction from beef.
Sure, a bison rib roast has the same fat cap as beef, and there's the same line of fat that runs between the spinalis (aka the ribeye cap, aka the tastiest cut on the steer) and the longissimus (the big eye of meat), but we're talking marbling here—the intramuscular fat that shows up in fine, spider-webbed patterns within the meat of a grain-finished steer.
In a beef roast, that fat helps insulate (resulting in slower, more even cooking), lubricate (making the meat seem juicier, even if slightly overcooked or dry), and flavor the meat.
What does this mean for bison? It means that you've got to cook it extra carefully to guarantee that it stays tender, moist, and flavorful. Even more so than beef, gentle, even cooking is what's going to lead to the perfect roast, and I can't think of a better way to complement its mild, meaty flavor than with a bit (or, ok, a lot) of smoke from the grill.
Many recipes for rib roasts advise starting the roast over high heat in order to sear its surface, then finishing it gently until it cooks through to the center, the thought being that searing somehow "locks in juices." But you and I both know that's a load of bison-crap, right? We've even done the experiments to prove it.
In reality, the best way to cook a large roast is to start it with slow, gentle heat, and finish it with a sear at the end.
See, the hotter you cook things, the greater the temperature differential that arises between the edges and the center. By blasting a roast with high heat, you very easily manage to overcook the exterior layers of meat long before the center has a chance to come up to the desired final temperature. With a beef roast, it's bad enough, but that extra fat can go a long way to mitigating any dryness. With bison, it's downright disastrous, leading you to dry, overcooked bites.
Cook it gently by starting it over the cooler side of a grill, and you can manage to get the center to that target 125°F medium-rare without the risk of overcooking the exterior.
To do this, I set half the burners of a gas grill to low to medium-low heat, leaving the rest shut off (or set up a mid-sized fire under one side of a kettle-style coal grill). Cooking with the lid closed, the grill should maintain a temperature of around 225°F, leading to a roast that takes about three hours to cook to completion—plenty of time for the meat to absorb some of the smokiness I've added in the form of a foil pouch of soaked wood chips
When to finally hits your target temperature, you may notice that, well, it's not particularly attractive—the exterior will be a muddy, pinkish gray color. This is because the browning reactions that add flavor and texture to the exterior of a good roast or steak don't take place at very low temperatures.
But here's the good news: since the meat is already relatively warm and has had a chance for its surface to dry out a bit, it can take on those colors and flavors much more quickly than a raw roast could, meaning plenty of browning with very little overcooked meat underneath.
I remove the roast from the grill after it comes to temperature, then bang the heat up as high as it will go (pile on the coals if using a coal grill!). After letting the grill preheat (lid closed for a gas grill to retain heat, lid ajar for a coal grill for better ventilation), the roast goes back on just long enough to brown it.
What you end up with is a perfectly evenly roasted bison rib with meat with plenty of smoky flavor, and a great crisp crust. The best part? The meat will be so juicy and moist that you won't even notice that the fat's not there.