In a large bowl, combine the zucchinis, red and yellow bell peppers, red onion, and mushrooms.
In a small bowl, whisk together the balsamic vinegar, olive oil, minced garlic, Italian herbs, salt, and pepper. Pour this mixture over the vegetables, tossing well to ensure they are evenly coated. For enhanced flavor, let the vegetables marinate for about 30 minutes
Preheat the grill to medium-high. Place the grill pan over the heat to allow it to also preheat
Once the pan is hot, arrange the vegetables in a single layer. You may need to work in batches. Try not to over-crowd the pan.
Grill the vegetables for 4 minutes each side or until they are tender and have grill marks. Use tongs to turn them for even grilling.
Transfer the vegetables to a serving platter. Adjust seasoning with additional salt and pepper if needed.
Garnish with fresh herbs; optional
Notes
If vegetables begin to stick or look dry, drizzle with a bit of olive oil and balsamic vinegar for moisture and flavor.
Different vegetables cook at different rates. Add longer-cooking vegetables (like onions and peppers) first, followed by quicker-cooking ones (like mushrooms and zucchini) later on.