Preheat a grill to medium high. Form the beef into four 6-ounce patties, about 3/4 inch thick; press the center of each patty with your thumb to make a 1/2-inch-deep indentation. Season the patties on both sides with salt and pepper. Lightly brush the grill grates with vegetable oil, then grill the patties, indentation-side up, until marked on the bottom, about 5 minutes. Flip and cook until marked and slightly firm, about 3 more minutes for medium doneness. During the last minute of grilling, brush the patties with the hoisin sauce. Remove the patties to a plate and let rest 5 minutes.
Meanwhile, brush the scallions with sesame oil, season with salt and grill, turning occasionally, until charred and tender, 3 to 4 minutes. Cut into 4-inch pieces. Grill the buns, cut-side down, until lightly toasted, about 1 minute.
Mix the mayonnaise, Sriracha and sesame oil in a small bowl and spread on the buns. Top the bun bottoms with the cucumber, burger patties and scallions, then cover with the bun tops.
Photograph by Levi Brown
Recipe courtesy Food Network Magazine