1/4 cup plus 2 tablespoons nonfat or light mayonnaise
1 tablespoon Chinese or honey mustard
6 light wheat or multigrain burger buns
6 slices red onion
6 lettuce leaves
Place the mushrooms, scallions, and red bell pepper in the bowl of a food processor and process until the vegetables are finely chopped.
Place the vegetables, ground turkey, teriyaki sauce, and black pepper in a medium bowl and mix thoroughly. Shape the mixture into six 4-inch patties.
Grill the burgers over medium coals or cook under a broiler for about 6 minutes on each side, until the internal temperature of the patties reaches 165 degrees and the meat is no longer pink inside. (To retain moisture, avoid pressing down on the patties as they cook.) Alternatively, coat a large nonstick skillet or griddle with nonstick cooking spray and cook the burgers over medium heat for about 5 minutes per side.
If desired, brush the pineapple slices with a little teriyaki sauce and grill or broil for a minute or two on each side, until lightly browned.
To assemble the burgers, place the mayonnaise and mustard in a small bowl and stir to mix well. Spread some of the mayonnaise mixture on each bun.
Place each burger in a bun and top with some of the onion, lettuce, and if desired, a pineapple slice. Serve hot.