Whisk all of the ingredients in a small bowl until well combined.
Marinate the mushroom caps in the marinade, and using a basting pastry brush, brush the marinade over them so that they're well coated.
Spray your grill pan with oil, and place on medium heat.
Once it's hot, place the marinated portobellos cap side down. As the mushrooms are being grilled, brush over the leftover marinade to get more flavor. Cook for 3-4 minutes, and you'll notice that grill marks will become obvious on the mushroom caps.
Flip them over and cook for 3 more minutes. Remove from the grill pan and serve.
Notes
To clean the mushrooms, use a clean paper towel and gently rub off any dirt from the caps. I don't recommend removing the gills as they add a lot to the flavor.
Remove the stalks for easier grilling. Using your fingers gently twist and pull the stalks and they should come right off. For super fresh mushrooms, you'll have to use a small paring knife to help you remove the stalks.
I recommend enjoying these grilled Portobello mushrooms on the same day, but if you have leftovers then store them in an airtight container in the fridge for up to 2 days.
You can reheat the leftover Portobellos if you like, but I serve them cold the next day.
I like to use olive oil for its flavor, but feel free to substitute it with a neutral oil with a higher smoke point.