In a bowl, combine the balsamic vinegar, olive oil, soy sauce, minced garlic, onion powder, paprika, ground cumin, smoked paprika, black pepper, and optional cayenne pepper. Stir well to blend the flavors.

Place the Portobello mushrooms in a shallow dish, gill side up. Pour the marinade over them, making sure they’re well-coated. Let them marinate for at least 15 minutes to absorb all the delicious flavors.

While the mushrooms are soaking in the marinade, preheat your grill to medium-high heat.

Carefully place the marinated Portobello mushrooms on the grill. Grill for about 5 minutes on each side or until they become tender and develop a delightful char.

Remove the mushroom steaks from the grill and serve them while hot.