Mizuna Salad with Shaved Gruyere and Roasted Portabellas

3/4 pounds sliced portabella mushrooms 1/2 cup extra-virgin olive oil, divided 3 tablespoons red-wine vinegar 1 teaspoon Dijon mustard 10 cups mixed spicy greens such as mustard, arugula, tatsoi, mizuna, and watercress 1 cup shaved gruyere cheese Preheat oven to Read on!