Preheat the oven to 375 degrees F. Lightly beat 1 large egg white with 1 tablespoon water. Brush on buns, then add toppings and bake until the egg white is dry, 6 to 8 minutes.

Nori-Sesame buns (pictured above): Brush with egg wash, then sprinkle with sesame seeds and thinly sliced roasted nori (seaweed sheets). Bake as directed.

Photographs by Jon Paterson/Studio D