Slice zucchinis into round circles, slice bell pepper into thick chunks, slice mushrooms into thick slices, rough chop red onion. Place all prepped veggies in a large mixing bowl.
Drizzle olive oil and balsamic over veggies. Sprinkle with salt, garlic powder, black pepper, and Italian seasoning. Toss veggies making sure they are well coated with oil, balsamic and spices.
Lightly oil a grilling basket and place on preheated grill. Place veggies in grilling basket. Grill veggies for 20 minutes on a medium flame. Occasionally tossing them by using a tong. This will make sure veggies are grilled evenly.
Use oven mittens and carefully remove basket from grill. Place veggies in a serving bowl and serve. Place any leftover veggies in an airtight container and keep refrigerated for up to 3-4 days.