In a small bowl, whisk together the balsamic vinegar, olive oil, minced garlic, dried thyme, salt, and pepper.

Place the Portobello mushrooms in a shallow dish and pour the marinade over them, making sure to coat both sides. Let them marinate for about 10 minutes.

Preheat the grill to medium-high heat.

Remove the mushrooms from the marinade and place them on the grill. Cook for about 5 minutes on each side, or until they are tender and have grill marks.

Serve the grilled Portobello mushrooms as a side dish or on a bun as a burger.