1. Combine first 3 ingredients, 2 tablespoons onion, 1 teaspoon mint, and 1/4 teaspoon pepper in a small bowl. Cover; chill until ready to use.
2. Combine lamb, remaining 2 teaspoons mint, remaining 1/4 teaspoon pepper, and next 3 ingredients.
3. Heat a large nonstick skillet over medium-high heat; coat pan with cooking spray. Add lamb mixture and remaining 2 tablespoons onion; cook 7 minutes or until browned, stirring to crumble. Stir in feta; cook 1 minute or until cheese begins to melt.
4. Line each pita half with 1/4 cup spinach and 3 tablespoons cucumber sauce; fill with about 1/2 cup lamb mixture and 1/4 cup tomato.