In a bowl or your food processor, mix the grated cheese, mayonnaise, pimentos, grated onion, and a few good grinds pepper until blended. Resist the urge to dig in immediately. Instead, cover it and stash it in the fridge for at least a couple hours and, preferably, 24 hours. (Trust us, the pimento cheese is unspeakably better after it rests. You can refrigerate it for up to 4 days, provided you can resist it that long.)