Meanwhile, in a small bowl combine the yogurt, mayonnaise, pepperoncini, pepperoncini liquid, lemon juice, garlic, and salt and pepper to taste. Set aside.
Once the spinach has cooled, remove it from the refrigerator and add the feta, oregano, ground turkey, 1/4 teaspoon salt and ground black pepper. Mix well, then shape into 4 patties, each about 1/2-inch thick.
Spray the burgers lightly with olive oil cooking spray, then grill over medium heat until just cooked through, about 6 minutes per side. Spread some of the yogurt sauce on the bottom half of each bun, then top with a burger. Spoon the remaining sauce over the burgers and top with the remaining bun hales. Serve immediately.
Nutrition information per serving: 460 calories; 220 calories from fat (48 percent of total calories); 24 g fat (7 g saturated; 0 g trans fats); 110 mg cholesterol; 34 g carbohydrate; 5 g fiber; 6 g sugar; 29 g protein; 1030 mg sodium.