Prepare marinade in a small bowl by whisking together balsamic vinegar, olive oil, lemon juice, mustard, and salt.
Remove stems from mushroom caps and wipe clean with a damp paper towel. Place mushrooms in a large resealable plastic bag. Pour in marinade, seal bag, and toss to coat thoroughly. Let marinate for at least 30 minutes, turning halfway through for even coverage.
Coat fryer with some oil to prevent sticking. Add portobellos to air fryer basket and cook at 370 degrees for 7 minutes.
Meanwhile... prepare your toppings of choice. Combine pesto and mayo, sauté some spinach, slice some tomatoes and red onions.
Once cook time is complete, remove mushrooms and allow to rest on separate plate for a few minutes. (They will give off quite a bit of liquid, so don't place it on a bun too quickly or your buns will go soggy.)
For this recipe, do not clean mushrooms with running water. Mushrooms are full of water and they also absorb water. But if you rinse them with water, they will not be able to absorb the marinade - which is where they will get their flavor. Instead, wipe them clean them with a damp paper towel.
Can I leave out the oil? While technically you can leave it out, the result will taste much different. There will be no fat, which will change the flavor and make the meal less satisfying.What type of air fryer can I use? Any of the air fryers on the market should work. Our favorite is the Instant Pot Duo Crisp, which is both a pressure cooker and air fryer.