Place the chicken pieces in a large Ziploc bag or bowl. Place the zucchini and red onion in a separate Ziploc bag or bowl. Set aside.
Zest one of the lemons and place the zest in a medium bowl. Juice both lemons and add to the lemon zest. Add the minced garlic, thyme, rosemary, olive oil, salt and pepper. Whisk the marinade together. Pour half of the marinade into the freezer bag or bowl with the chicken pieces and pour the other half in the freezer bag or bowl with the zucchini and onion. Let marinade for 30 minutes or up to 4 hours in the refrigerator.
When ready to grill, make the kebabs. Alternate chicken, zucchini, and onion on skewers. Discard any remaining marinade.
Lightly brush the grill with olive oil and preheat to medium heat.
Grill chicken kebabs, turning often so each side browns and has light grill marks, about 10-12 minutes or until chicken is cooked through. Serve immediately.
Note-If using wooden skewers, soak them in water before using. If using metal skewers, no prep is necessary.