1. Marinate the chicken for 10 minutes or up to eight hours in your marinade.
2. Preheat grill to high heat (at least 450 degrees Fahrenheit/220 degrees Celsius).
3. Spray grill grates with oil and grill onions and chicken over high heat for seven to 10 minutes per side, or until chicken juices run clear. Remove from grill and let chicken rest for five minutes.
4. Combine mayo, garlic, chives, honey, water, and salt for the garlic mayo.
5. Assemble the sandwich: Spread garlic mayo on the lower bun, then add arugula, chicken, grilled onion, and top bun.