Turkey-Spinach Burgers with Pepperoncini Sauce

Ingredients

• 2 Tbsp extra-virgin olive oil, divided, plus olive oil cooking spray
• 1/2 cup finely chopped onion
• 5 oz baby spinach
• Kosher salt
• freshly ground black pepper
• 1/4 cup plain nonfat Greek yogurt
• 1/4 cup low-fat mayonnaise
• 2 Tbsp minced seeded pepperoncini
• 1 Tbsp liquid from the pepperoncini jar
• 1 to 2 tsp fresh lemon juice or to taste
• 1 tsp minced garlic
• 3 oz crumbled feta cheese
• 2 Tbsp chopped fresh oregano
• 1 lb ground turkey
• 4 whole-wheat hamburger buns, toasted
• Lettuce and sliced tomatoes for garnish