If using wooden skewers, soak them in water until ready to use.
In a large bowl, gently mix together the marinade ingredients: olive oil, balsamic vinegar, salt, pepper, garlic powder, and oregano.
Wipe the mushrooms clean with a damp paper towel. If they have prominent stems, remove them.
Cut the bell pepper and red onion into squares.
Slice the zucchini into ¼-inch thick slices.
Add the prepared vegetables to the marinade and gently mix until well-coated.
Heat your grill on medium-high. I use an indoor dual-contact grill. You can use a similar grill, an outdoor grill, or a grill pan.
Thread the vegetables on six skewers.
Oil your grill and place the skewers on it. Grill the vegetables until tender, 4-5 minutes per side, or 3 minutes per side if using a dual-contact grill.
Serve immediately. They are good hot or at room temperature.
Video
Notes
You can prep the veggies, coat them in the marinade, and thread them on skewers one or two hours before you grill them. Keep them in the fridge until you're ready to use them.
You can vary the veggies you use. Good candidates include yellow onion, yellow squash, and cubed portobello mushrooms. You can also use different spices. Two spices that I like to add are onion powder and ground cumin. I add a generous pinch of each.
I prefer garlic powder to fresh minced garlic in this recipe. It better coats the veggies, and it doesn't burn on the grill.
You can keep the leftovers in the fridge, in an airtight container, for up to 4 days. Reheat them gently in the microwave, covered. They are also good cold, like antipasti.