1. Heat oil in a skillet over high heat. Add mushrooms; sauté 4 minutes. Add herbs and garlic; sauté 1 minute. Stir in 1/3 cup broth, juice, and pepper.
2. Bring milk and 1 1/2 cups broth to a boil. Stir in polenta; cook 4 minutes, stirring constantly. Stir in half of cheese and 1/4 teaspoon salt. Divide polenta among 4 gratin dishes; top with remaining cheese. Broil 5 minutes. Top each serving with 1/2 cup mushrooms.