In a medium sized bowl, mix together all marinade ingredients.
Add chicken breasts to marinade. Cover and refrigerate. Allow to marinate for at least 30 minutes.
Preheat grill to high heat.
Preheat oven to 350 degrees F.
When ready, remove chicken from marinade. Place on a grill and cook on high heat for 6 minutes on each side. Remove from grill to a heat-proof pan and place in the oven for 10 minutes, to finish cooking through.
Dice the cucumber and add to a medium bowl. Stir in the remaining ingredients and combine well. Cover and place in refrigerator until ready to use.
Slice chicken into strips and place over a bed of spinach, if desired, on a serving platter. Top with herb-yogurt-cucumber sauce and serve.