Make the marinade: In a large bowl, whisk together olive oil, lemon juice (and zest if using), garlic, honey, smoked paprika, oregano, salt, pepper, and red pepper flakes.
Prep the chicken and veggies: Pat the chicken dry and cut into even pieces.
Chop the peppers, onion, and zucchini into similarly sized chunks so they cook at the same rate. Keep mushrooms whole if small; halve if large.
Marinate: Add the chicken to the bowl and toss to coat. Let it sit 20–30 minutes at room temperature, or cover and refrigerate up to 4 hours.
Reserve 2–3 tablespoons of marinade separately to brush onto the veggies.
Toss the veggies: In another bowl, combine peppers, onion, zucchini, mushrooms, and tomatoes (if using). Drizzle with the reserved marinade and a pinch of salt. Toss gently.
Preheat the grill: Heat a gas or charcoal grill to medium-high (about 400–425°F).
Clean and oil the grates to prevent sticking.
Assemble the kabobs: Thread chicken and vegetables onto skewers, alternating pieces. Don’t pack too tightly—leave small gaps for even heat circulation.
Grill: Place kabobs on the grill. Cook 10–14 minutes, turning every 3–4 minutes, until the chicken reaches an internal temperature of 165°F and has light char marks.
Brush lightly with any remaining marinade during the last couple of minutes if desired.
Rest and finish: Transfer kabobs to a platter and rest 3–5 minutes. Sprinkle with chopped parsley. Serve with lemon wedges for a fresh squeeze over the top.
Optional sides: Pair with rice, couscous, grilled flatbread, or a crisp salad.