In a large bowl, whisk yogurt, tandoori seasoning and, if desired, pepper flakes until blended. Add chicken and turn to coat. Cover and refrigerate 6-8 hours, turning occasionally.Preheat oven to 450°. Drain chicken, discarding marinade in bowl. Place chicken in a greased 15x10x1-in. baking pan. Add sweet potatoes; drizzle with oil. Bake 15 minutes. Add tomatoes and lemon slices. Bake until a thermometer inserted into chicken reads 170°-175°, 10-15 minutes longer.
Broil 4-5 in. from the heat until browned, 4-5 minutes. If desired, serve with cilantro and naan.
2 chicken thighs with 1 cup sweet potatoes and 4 tomatoes: 589 calories, 27g fat (9g saturated fat), 186mg cholesterol, 187mg sodium, 29g carbohydrate (13g sugars, 6g fiber), 52g protein.