Greek Style Lamb Pita Sandwiches

Ingredients

• 3/4 cup plain Greek yogurt
• 1/2 cucumber, peeled, halved lengthwise, seeded, and chopped fine
• 2 tablespoons lemon juice
• 3 garlic cloves, minced
• 1 tablespoon minced fresh mint or dill
• Salt
• pepper
• 4 (8-inch) pita breads
• 1/2 onion, chopped coarse
• 1 tablespoon minced fresh oregano or 1 teaspoon dried
• 1 teaspoon ground cumin
• 1 pound ground lamb
• 1 teaspoon vegetable oil
• 1 large tomato sliced thin
• 2 cups shredded lettuce
• 2 ounces feta cheese crumbled (1/2 cup)