1. In a large container or flat baking tray, combine all the marinade ingredients. For added flavour, you can add extra spices or double the ingredients.
2. Remove the stems from the Portobello mushrooms. Rinse them thoroughly and pat them dry using paper towels or a cloth.
3. Submerge the Portobello mushrooms in the marinade for 20-30 minutes, flipping them halfway through the process.
4. Grill the mushrooms over high heat. Check on them after 10 minutes, then flip and cook for 5-10 more minutes. (I had to cook mine a bit longer because my grill wasn't hot enough.)
5. Place a grilled Portobello mushroom on a bun and add your preferred burger toppings such as lettuce, tomato, pickles, onion, and mustard.