When I am cooking the portobello I season it with dried herbs, salt, and pepper. But you don't have to stop there. You can season the mushroom with the same seasoning you would for any burger. You can make it taco-style with chili and cumin, or just add some onion powder and garlic powder to the spice blend. Of course, you can elevate the listed seasoning with some oregano and/or basil for a more Italian-style flavor profile.