In a large bowl, mix the olive oil, balsamic vinegar, Dijon mustard, and garlic.

Add the portobello mushroom caps to the mixture, and mix well to cover all surfaces.

Season with salt and pepper, cover the bowl, and marinate for at least 30 minutes.

Remove the mushrooms from the marinade, and cook on a grill, grill pan, or in a regular skillet over medium-high heat for 7 to 10 minutes on each side, until tender, and serve.