My Big Fat Greek Burger

Ingredients

• 1 lb 93 % lean ground beef
• 1/2 cup cooked spinach , squeezed dry ( frozen works well )
• 1/2 cup crumbled reduced-fat feta cheese , preferably sheep's-milk
• 2 teaspoons chopped fresh dill
• 1 teaspoon chopped fresh oregano
• 1 teaspoon ground cumin
• 1 teaspoon minced garlic
• 3/4 teaspoon kosher salt
• 1/2 teaspoon fresh ground pepper
• 1/2 cup nonfat greek-style plain yogurt
• 1 teaspoon freshly grated lemon zest
• 2 teaspoons lemon juice
• 1 pinch kosher salt
• fresh ground pepper
• 4 whole wheat pita bread or 4 white pita bread , warmed
• 1/2 cucumber , cut into 16 thin slices
• 2 plum tomatoes ( use a dry tomato to avoid a soggy sandwich ) or 3 campari tomatoes , thin sliced ( use a dry tomato to avoid a soggy sandwich )
• 1 small red onion , cut into thin papery slices