6 cloves garlic, unpeeled
1/4 cup extra-virgin olive oil, divided, plus 3/4 cup olive oil
1/3 cup balsamic vinegar
1 tablespoon Dijon mustard
4 large Portobello mushrooms, wiped of any dirt, washed and quickly dried
Preheat the grill. Preheat the oven to 400 degrees.
In a small bowl, toss the garlic cloves with 2 tablespoons of the olive oil and salt. Wrap the garlic cloves in tinfoil and place on the tray in the center of the oven. Cook until tender when pierced with the tip of a knife, 20 to 25 minutes.
In a medium bowl, whisk together the balsamic with the 3/4 cup olive oil and mustard.
In a medium bowl, toss the mushrooms in the remaining 2 tablespoons of olive oil. Season with salt. Place them, stem side up, on the hottest part of the grill. Cook for 3 to 4 minutes, you may see liquid starting to emerge from the stem area. Turn on the other side and cook for an additional 3 to 4 minutes. Move to a cooler side of the grill and continue cooking until soft when pierced with a fork. Make sure to move them to an indirect heat part of the grill so that you do not burn the outside of the mushroom but still cook it through. Reseason them lightly with salt.
Remove the garlic from the oven. Squeeze the garlic pulp out from their skins and whisk it vigorously into the dressing. Arrange the mushrooms on a serving platter and top with the dressing. Serve immediately.