GRILLED MARINATED MUSHROOMS 1 tablespoon olive oil 2 tablespoons balsamic vinegar 1 teaspoon dried oregano 8 ounces baby portobello mushrooms, halved lengthwise Whisk the olive oil, balsamic vinegar, and oregano together in a medium bowl. Add the halved mushrooms and toss well to coat. Marinate for 30 minutes. Thread the mushrooms onto skewers. Roast them in the oven for 10-12 minutes, or grill them, 5 minutes per side.