Can I make this in a regular oven instead of an Air Fryer?

Yes, bake the marinated Portobello mushrooms at 200°C (400°F) for about 15-18 minutes, flipping halfway through. They won't get quite as crisp as in the Air Fryer, but the flavor stays the same.

What can I use if I don't have Portobello mushrooms?

Large flat-cap mushrooms or king oyster mushrooms work well too - just pick pieces wide enough to cover the bun. Slice king oyster mushrooms lengthwise into thick "steaks" instead.

How do I keep the burger from getting soggy if I prep it ahead?

Assemble the burger just before eating; the mushrooms and caramelized onions can be cooked up to a day ahead and reheated separately. Keep the bun, sauce, and toppings separate until serving to preserve the crunch.