I found the recipe here: - - - > http://www.seriouseats.com/recipes/2012/09/balsamic-marinated-portobello-burgers.html
1/4 cup balsamic vinegar
1/4 cup olive oil
2 tablespoons juice from 1 lemon
1 tablespoon Dijon mustard
3 medium cloves garlic, minced (about 1 tablespoon)
1 teaspoon finely minced thyme leaves
1 teaspoon Kosher salt
1. In a small bowl, whisk together balsamic vinegar, olive oil, lemon juice, mustard, garlic, thyme, and salt.
2. Place portobellos in a large resealable plastic bag. Pour in marinade, seal bag, and toss to coat mushrooms thoroughly. Let marinate at room temperature for 30 minutes.