Creating your own Portobello Mushroom Burger is simple when you follow these clear steps:

1. Preheat the Grill: Heat your grill or grill pan over medium heat.
2. Prepare the Marinade: In a small bowl, whisk together the olive oil, balsamic vinegar, garlic powder, onion powder, salt, and pepper.
3. Marinate the Mushrooms: Brush the marinade generously over both sides of the Portobello mushrooms.
4. Grill the Mushrooms: Place the mushrooms on the grill, gill side down. Grill for about 5-7 minutes, then flip and grill for another 5-7 minutes until tender.
5. Melt the Cheese: If using cheese, place a slice on top of each mushroom during the last minute of cooking to melt.
6. Toast the Buns: While the mushrooms are finishing, toast the burger buns on the grill for about 1-2 minutes until golden.
7. Assemble the Burger: On each toasted bun, layer lettuce, the grilled Portobello mushroom, tomato slices, and avocado if desired.
8. Add Sauce: Drizzle on ketchup or your preferred sauce to finish.
9. Serve: Top with the other half of the bun and serve immediately.

Following these steps will lead to a deliciously satisfying meal that highlights the unique flavors of the Portobello mushroom.