Remove large stems from the mushrooms, wash with cold water, and let mushrooms drain in a colander while you cut other ingredients.
Cut off both ends of the onion, and then cut the onion into chunks that are about the same size as the mushrooms. (Try to keep the onion layers together if you can.)
Cut the cheese into pieces the same size as the mushrooms and onions.
Whisk together the lemon juice, red wine vinegar, olive oil, Greek Seasoning (affiliate link), and Spike Seasoning (affiliate link) to make the marinade.
Put the mushrooms, onions, and cheese pieces into a Ziploc bag and pour in the marinade. Let ingredients marinate in the refrigerator for a few hours.
When you’re ready to grill, make sure the grill surface is clean and then pre-heat to medium high while you thread mushrooms, cheese, and onions on skewers.
Spray the grill with non-stick grilling spray or brush with a high smoke-point oil right before you put the skewers on. (Be careful in case it flames up.)
Grill skewers about 6 minutes per side, using a flat turner to scrape the cheese away from the grill if needed when you’re ready to turn them. Serve hot.
