Soak 8-12 wooden skewers in water for at least 20 minutes (if using metal skewers, no soaking required).
Preheat grill to medium-high heat (350°F to 450°F).
Cut all vegetables into uniform-sized pieces to ensure even cooking.
Thread vegetables alternately onto skewers: onion, zucchini, bell peppers, mushrooms, and cherry tomatoes.
In a small bowl, whisk olive oil, salt, pepper, Italian seasoning, and garlic (if using).
Brush the assembled skewers generously with the olive oil seasoning mixture.
Place skewers on the preheated grill. Cook for 12-15 minutes, turning every 3-5 minutes until vegetables are tender and slightly charred.
Remove from grill and serve immediately.