Place egg white in a large bowl; lightly beat with a whisk. Add onion and next 6 ingredients (through turkey); stir well to combine. Divide turkey mixture into 4 equal portions, shaping each into a 1/2-inch-thick patty.
Heat a large nonstick skillet over medium-high heat. Coat pan with cooking spray. Add patties to pan; cook 8 minutes on each side or until done.
Place patties on bottom halves of hamburger buns. Divide peppers evenly among burgers; cover with top halves of buns.
Note: To freeze extra servings, wrap each uncooked patty individually first in plastic wrap and then in heavy-duty aluminum foil. Freeze for up to 2 months. To serve, thaw patties completely in refrigerator, and then cook and serve as directed.