These mushroom burgers are made exclusively with the caps of Portobello, no beef involved, which means they're an excellent recipe to use up your bounty of fresh mushrooms.
Combine all marinade ingredients in an immersion blender. Blend until smooth.
Place mushroom caps in a shallow dish upside down (gill side up). Pour marinade over the mushroom caps and let sit for 1-2 hours.
In a medium bowl, add red onion and baby Bella mushrooms. In a small bowl combine lemon pepper, garlic salt, and olive oil. Pour olive oil mixture over the onions and mushrooms. Toss to coat well.
Preheat grill to 350 F. Also, preheat a skillet (or the flat top side of your grill if you have one).
Place mushroom caps upside down onto hot grill. Cook for 2-3 minutes, flip over with spatula and cook an additional 2-3 minutes.
While the mushroom caps are cooking, add the onion/mushroom mixture to the skillet or flat top. Cook, turning frequently, until tender, about 4-5 minutes.
Move mushroom caps, right side up, to the flat top. Add a slice of Mozzarella cheese to each cap, then top with the mushroom/onion mixture, followed by a second slice of cheese.
Toast buns and use a spatula to move cheese topped mushroom caps to the buns.
Notes
It's not necessary to remove the gills when making these burgers, but if that's how you prefer it then go right ahead. Consuming Portobello mushroom gills is perfectly safe and they are typically only removed to prevent them from turning the surrounding ingredient a muddy color, which we don't have to worry about with these burgers. Simply scoop out the gills with a spoon if preferred.
We use a grate and flat top combination grill. We find it to be an invaluable tool for outdoor cooking allowing us so much versatility! Alternatively, many gas grills have a side burner that allows you to cook something in a skillet while cooking on the grill. You can also place a cast iron skillet directly on your grill grates, just be sure to use a pot holder! Finally, you can cook everything in the house by using a countertop indoor grill or a grill pan.