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Place all marinade ingredients in a resealable plastic bag, mix well, and marinate in refrigerator at least 2 hours.
In a bowl, prepare sauce by combining mayonnaise, yogurt, mint, cucumber, and chives. Add salt and pepper and set aside.
Preheat BBQ on high. On a well-oiled grill, cook chicken on indirect heat by turning off burner directly under chicken and leaving opposite burner on high. Grill chicken breasts at least 6 minutes on each side until cooked through. Let chicken rest 5 minutes, then cut diagonally into slices.
Oil bread slices and toast on BBQ until crisp and golden brown.
Assemble sandwiches by spreading yogurt sauce, adding a few slices of avocado and chicken, and garnishing with fresh herbs and pistachios.