In a large mixing bowl, whisk together the olive oil, vinegar, onion seasoning, and garlic.
To the marinade, add in the chopped zucchini, squash, peppers, onions, and tomatoes if using. Do NOT add in the mushrooms. Toss until the vegetables are coated in the marinade. Cover and refrigerator for at least 1 hour, but overnight is best.
If using wooden skewers, place in a large rectangular baking dish and cover with water. Let soak for at least 1 hour, or overnight as well. If using metal skewers, skip this step.
Once the vegetables have marinated, thread them onto the soaked skewers, in an alternating pattern, adding in the cleaned mushrooms to the mix.
Preheat the grill or grill pan to medium heat. Once heated, place the prepared skewers onto the grill and cook for 5 minutes. Rotate the skewers and grill for another 4-6 minutes, or until the vegetables are tender.
The variety of vegetables is up to you. Keep in mind that potatoes would need to be parboiled BEFORE marinating and grilling to be cooked through. For Dried Onion Soup Mix: Use 2 teaspoons onion flakes, 1 teaspoon salt, and ½ teaspoon each of garlic powder, onion powder, celery powder, pepper, and dried parsley. Alternatively, use 1 teaspoon kosher salt, 1 teaspoon onion powder, and ½ teaspoon pepper. Do not be tempted to add the mushrooms to the marinade. They will soak up the liquid and get soggy and have a funny texture. Leftover grilled vegetables can be stored in the refrigerator for up to 4 days and are delicious in salads, pasta, or frittatas.