In a medium bowl, whisk together the mustard, vinegar and herbes de provence. Whisk in 2 tbsp. EVOO. Reserve 1 tbsp. of the marinade for the vegetables. Add the chicken to the bowl with the remaining marinade and coat well using a rubber spatula. Let marinate for at least 30 minutes or up to 12 hours.
Preheat a gas grill to 400 degrees . Whisk the remaining 2 tbsp. EVOO and 1/8 tsp. each salt and pepper into the reserved marinade. Add the zucchini and onion and toss to coat.
Season the chicken with 1/2 tsp. each salt and pepper. Onto each of eight 12-inch skewers, thread 1 piece of chicken, 1 zucchini slice, a section of onion, a lemon wedge and a second piece of chicken; repeat with the remaining vegetables and chicken.
Brush the hot grill grate clean, then lightly oil. Add the kebabs crosswise across the grate, cover the grill and cook for 3 minutes. Using a sturdy metal spatula, turn the kebabs over, releasing the meat from the grate and rolling away from you. Cover and cook until the meat is firm and browned all over, about 5 minutes.