1. Place bulgur with contents of seasoning packet in large bowl. Add boiling water; cover with plastic wrap; let stand 30 minutes.
2. Cut cucumber in half lengthwise; scoop out seeds. Coarsely grate one of the halves; squeeze out excess liquid. Stir together grated cucumber, yogurt, garlic and 1/4 teaspoon salt in small bowl. Cover; refrigerate for 20 minutes or up to 2 days.
3. Coarsely chop the remaining cucumber; add to bulgur, along with tomatoes, green onion, mint, olive oil, lemon juice, remaining 1/2 teaspoon salt and the pepper. Let stand 10 minutes, or cover and refrigerate up to 2 days.
4. To serve, spread tabbouleh on platter. Top with chicken; spoon yogurt sauce over chicken. Garnish with lemon slices if desired. Makes 4 servings.