Clean the portobello mushroom caps by wiping them gently with a damp paper towel. Remove the stems and use a spoon to carefully scrape out the dark gills from the underside. Removing the gills prevents excess moisture and gives a cleaner presentation.
In a small bowl, whisk together the balsamic vinegar, extra virgin olive oil, minced garlic, italian seasoning, salt, black pepper, and red pepper flakes.
Place the mushroom caps in a shallow dish gill-side up and spoon the marinade over them, making sure it pools into the cavities. Let them marinate for 10 to 15 minutes, spooning the marinade over once or twice during that time.
Preheat the air fryer to 375 degrees F. Place the mushroom caps in the basket gill-side up. Spoon any remaining marinade from the dish over the tops.
Air fry for 8 to 10 minutes, until the mushrooms are tender, juicy, and slightly caramelized around the edges. The caps should be soft when pressed gently but still holding their shape.
Transfer to a serving plate and garnish with fresh chopped parsley. Serve these as a hearty side dish, slice them for sandwiches, or use them as a burger substitute on toasted buns.