Prep the chicken: If using chicken breasts, pound them to an even 3/4-inch thickness so they cook evenly. Pat dry with paper towels and set aside.
Make the marinade: In a bowl, whisk 3 tablespoons olive oil, lemon juice, garlic, parsley, thyme or oregano, smoked paprika, cumin, red pepper flakes, 1 teaspoon salt, and 1/2 teaspoon black pepper.
Marinate: Add chicken to a zip-top bag or shallow dish.
Pour in half the marinade and coat well. Reserve the other half for the vegetables. Chill for 20–60 minutes.
If you’re short on time, even 10 minutes helps.
Prep the vegetables: Slice peppers into thick strips. Cut red onion into 1/2-inch wedges. Slice zucchini and yellow squash lengthwise into planks about 1/2 inch thick.
Leave cherry tomatoes whole. Halve mushrooms if large.
Season the vegetables: Toss vegetables with the remaining marinade and 2–3 tablespoons olive oil. Add another pinch of salt and pepper.
If using balsamic, add it now for a sweet, tangy finish.
Preheat the grill: Heat a gas or charcoal grill to medium-high (about 400–450°F). Clean and oil the grates with a folded paper towel dipped in oil, using tongs.
Grill the chicken: Shake off excess marinade. Grill chicken 4–6 minutes per side, depending on thickness, until the internal temperature reaches 165°F.
Move to indirect heat if browning too fast. Let rest on a plate, tented with foil.
Grill the vegetables: Place the sturdier vegetables (onions, peppers, zucchini, squash, mushrooms) on the grill first. Grill 3–5 minutes per side until tender with grill marks, but not mushy.
Add cherry tomatoes on a grill pan or skewers for 3–4 minutes, just until blistered.
Slice and serve: Slice the chicken against the grain. Arrange with the grilled vegetables on a platter. Squeeze fresh lemon over everything and sprinkle with extra chopped parsley.
Taste and adjust seasoning with salt and pepper.
Optional add-ons: Serve with a side of brown rice, quinoa, or a simple arugula salad. A light yogurt-garlic sauce or tzatziki pairs beautifully.