1. Preheat grill to high heat.
2. In a large bowl, combine the soy sauce, sesame oil, brown sugar, red pepper flakes, ginger, garlic, and sandwich bread. Stir, then let the mixture sit for a few minutes to allow the bread to soften. Mash the mixture with a fork into a paste (it won't get completely smooth, but try to get it as uniform as possible). Add the beef and scallions. Using your hand, mix until well combined. Divide the mixture into 5 equal portions, shaping each into loose balls. Flatten the balls into ¾-inch thick patties about 4½ inches around. Form a slight depression in the center of each patty to prevent the burgers from puffing up on the grill.
3. Place the patties on a grill rack coated with cooking spray. Grill, covered, until nicely browned on the first side, 2-4 minutes. Flip burgers and continue cooking for a few minutes more until desired doneness is reached. Before serving, toast the buns on the cooler side of the grill if desired.
4. Place 1 patty on bottom half of each bun; top each serving with lettuce leaves, chili-mayonnaise, and top half of bun.

For the Chili Mayonniase

1. In a small bowl, mix together the mayonnaise, Asian chili sauce, and sugar. (Note that this makes a spicy sauce; use less Asian chili sauce for a milder version.)
2. Make Ahead: The patties can be made a day ahead of time. Place them on a sheet pan lined with aluminum foil and cover tightly with saran wrap.
3. Freezer-Friendly Instructions: The uncooked burgers can be frozen for up to three months. (Freeze the burgers on a baking sheet or plate so their shape sets, then transfer them to a sealable plastic bag for easy storage.) Defrost overnight in the refrigerator prior to serving and then cook as directed.