The Montana bison burger is the rare burger where the cook has to fight the meat's own leanness rather than render its fat. Bison is markedly leaner than beef and slightly sweeter, which means the usual burger logic, sear hard and let the fat baste the patty, works against it: with little intramuscular fat to lose, an overcooked bison patty goes dry and crumbly fast. The defining decision is restraint at the heat. The patty is seared for crust but pulled well short of where a beef patty would be, kept at the rare-to-medium edge so the lean meat stays moist and its faint sweetness reads. Everything else is a standard burger build, a soft bun, cheese, the cool acidic frame, but the timing is not standard, and the timing is the sandwich.
It works because the rest of the build is arranged to cover for a protein that brings no fat of its own. The bun is soft and faintly sweet so it compresses to the patty rather than drawing moisture out of it. A slice of cheese does more structural work here than on a beef burger, draping the leaner patty in fat it cannot supply itself and sealing the surface against juice loss. Sauteed mushrooms, caramelized onion, or a slick of mayonnaise-based sauce show up often precisely because they put back richness the meat lacks. The pickle and raw onion are still the sharp, cold counter, but the patty they frame is cleaner and sweeter than beef, so the build leans a touch less acidic. A grill or a flat-top can both work; what cannot move is the rule that a lean patty cooked to a beef doneness is a failed patty.
The variations are mostly a question of how far the build compensates for the leanness. A blue cheese and onion version pushes richness hard; a plain version with only mustard and pickle trusts the meat and lets its sweetness stand alone; a blended patty cuts bison with beef fat to forgive the cook a little. Game-meat cousins, elk and venison builds, run the same problem with the same solution. Each of those is its own sandwich with its own balance and deserves its own article rather than being crowded in here.