Creating these Grilled Portabella Mushroom Burgers is a straightforward process when you follow these simple steps:

1. Marinate Mushrooms: In a large bowl, combine balsamic vinegar, olive oil, minced garlic, dried herbs, salt, and pepper. Coat the Portabella mushrooms in this marinade and let them sit for at least 30 minutes.
2. Preheat Grill: While the mushrooms are marinating, preheat your grill to medium-high heat.
3. Prepare the Buns: Lightly toast the burger buns on the grill for about 1-2 minutes until warm and slightly crispy.
4. Grill Mushrooms: Place the marinated Portabella mushrooms on the grill. Cook for about 5-7 minutes on each side or until tender and juicy.
5. Add Cheese: In the last minute of grilling, place a slice of mozzarella cheese on each mushroom to melt slightly.
6. Assemble Burgers: Take the grilled mushrooms off the grill and place them on the toasted buns. Top with fresh spinach, sliced tomatoes, and avocado if desired.
7. Add Sauce: Spread pesto or mayonnaise on the top half of the bun for added flavor.
8. Serve Warm: Close the burgers and serve immediately, enjoying them while warm for the best taste!

These steps will guide you through creating irresistibly delicious Grilled Portabella Mushroom Burgers that everyone will love.