Preheat a grill or broiler. Mix the ground beef with salt and pepper and form into 8 patties.
In a large bowl combine the lettuce, tomato, cucumber, olives, red onion and feta. Season with salt and pepper and toss with the lemon juice and olive oil.
Grill or broil the burgers 2 minutes per side for medium-rare. Toast the pita breads and cut off the top third. Stuff each with two burgers and salad and serve with the yogurt.