Beef Bulgogi Burgers - Once Upon a Chef

Ingredients

• 3 tablespoons soy sauce
• 1 tablespoon Asian/toasted sesame oil
• 1½ tablespoons (packed) dark brown sugar
• ¼ teaspoon crushed red pepper flakes
• ¼ teaspoon ground ginger
• 2 cloves garlic, (minced)
• 1 slice white or whole wheat sandwich bread, (crusts removed and torn into ¼-inch (6-mm) pieces)
• 1½ pounds 85% lean ground beef
• ½ cup chopped scallions, (from 4 to 5 scallions)
• 5 hamburger buns
• Lettuce leaves, (for serving (optional))
• ¾ cup mayonnaise, (best quality such as Hellmann's or Duke's)
• ¼ cup Asian chili sauce, (such as Sambal Oelek (available in Asian aisle of most supermarkets))
• ¼ teaspoon granulated sugar