Greek Pita Burgers with Spinach, Feta and Tzatziki Sauce

Sauce

• 1/2 medium cucumber, peeled, seeded and shredded
• 1 container (8 oz) plain yogurt
• 4 medium green onions, chopped (1/4 cup)
• 1 clove garlic, finely chopped
• 1 tablespoon olive oil
• 1 teaspoon lemon juice
• 1/4 teaspoon salt

Burgers

• 1 lb lean (at least 80%) ground beef
• 1/2 cup frozen chopped spinach, (thawed) squeezed to drain
• 1/3 cup crumbled feta cheese
• 1/4 cup finely chopped red onion
• 2 cloves garlic, finely chopped
• 1/2 teaspoon salt
• 1/4 teaspoon pepper
• 8 whole wheat hamburger buns, split
• 2 medium tomatoes, sliced
• 4 leaves romaine lettuce, torn in half