In a medium bowl, combine ground beef, oats, red onion, egg white, 2 tablespoons of the mint, and the garlic salt; mix well. Form into four 3/4-inch-thick patties.

Place patties on the rack of an uncovered grill directly over medium coals. Grill for 12 to 14 minutes or until done (160 degrees F),* turning once. Add pepper quarters to grill for the last 8 to 10 minutes of grilling or until skins are charred and pepper quarters are tender, turning once.

Meanwhile, in a small bowl, stir together yogurt and feta cheese.

Cut pepper quarters into strips, peeling off any loose skin if desired. Serve burgers with pita halves, pepper strips, yogurt mixture, and spinach. Sprinkle with the remaining 1 tablespoon mint. (If using hamburger buns, spread yogurt mixture onto bottoms of buns. Top bun bottoms with spinach, burgers, pepper strips, and remaining 1 tablespoon mint. Replace bun tops.) Makes 4 servings.

The internal color of a burger is not a reliable doneness indicator. A beef patty cooked to 160 degrees F is safe, regardless of color. To measure the doneness of a patty, insert an instant-read thermometer through the side of the patty to a depth of 2 to 3 inches.