Use a damp paper towel to wipe down the outside of each portobello cap. With a spoon, scrape out the dark gills from the underside of each cap and discard them. This step gives you more room to pack in the filling. Brush the outside of each mushroom lightly with olive oil using a pastry brush or your fingers.

Heat the remaining olive oil in a large skillet over medium-high heat. Add the diced onion and cook for 3 minutes until it softens and starts to turn translucent. Add the garlic and cook for 30 more seconds until fragrant.

Add the ground beef to the skillet. Break it up with a spatula and cook for 5 to 6 minutes until no pink remains. Drain any excess fat from the pan. Season with salt, pepper, Italian seasoning, and red pepper flakes if using.

Stir the marinara sauce into the beef mixture and cook for 1 more minute just to bring everything together and let the sauce coat the meat. Remove from heat.

Set each oiled portobello cap gill-side up in your air fryer basket. Spoon the beef mixture evenly into each cap, pressing it in gently so it is heaped but not spilling over. Top each one generously with shredded mozzarella.

Air fry at 370 degrees F for 12 to 15 minutes. The mushrooms are ready when the cheese is fully melted and browning in spots, the caps have softened and released a little liquid at the edges, and the filling is hot throughout. Check at 12 minutes. Large or very thick caps may need the full 15.

Use a spatula to lift each stuffed mushroom out of the basket carefully since they will be juicy at the bottom. Plate them, scatter torn fresh basil over the top, and serve alongside a simple green salad.