Prepare the Marinade: In a bowl, combine olive oil, balsamic vinegar, minced garlic, fresh thyme leaves, salt, and pepper. Whisk until well-blended.
Marinate the Portobellos: Gently brush each Portobello cap with the marinade, ensuring they are well-coated. Let them marinate for at least 30 minutes to allow the flavors to meld.
Preheat the Grill: While the mushrooms marinate, preheat your grill or grill pan to medium-high heat.
Grill to Perfection: Place the marinated Portobellos on the grill, gill side down. Grill for 4-5 minutes per side or until they develop a beautiful char and are cooked through.
Serve and Garnish: Plate your grilled Portobello mushrooms, sprinkle with fresh thyme leaves and optional Parmesan cheese. Serve them as a delightful side or as the star of your meal.