Soak approximately 8-12 wooden skewers in water for at least 15-30 minutes. The total number of skewers will vary depending on several factors, such as the size of each vegetable, the size of your vegetable pieces, etc.
Prepare the vegetables for the skewers. For each red onion, first peel, then slice in half. Chop each half into thirds, as if you were cutting a pizza (6 total pieces from one onion). Seed and chop each bell pepper into approximately 1-inch by 1-inch pieces. Finally, chop the zucchini into rounds.
Prepare the Garlic Herb Sauce- in a medium bowl whisk together the olive oil, minced garlic, parsley, cilantro, rosemary, salt, and pepper. Set aside.
Skewer the vegetables by alternating between red onion, zucchini, and the different colored bell pepper. Brush skewers with a light layer of olive oil.
Preheat grill to medium-high heat. Once the grill is hot, add skewers and grill for approximately 5-8 minutes per side, or until vegetables are just softening and browning around the edges.
Remove from heat, brush with garlic herb sauce and drizzle with balsamic vinaigrette, if desired. Enjoy!