Arrange mushrooms in a shallow glass dish. In a bowl whisk together lemon juice, vinegar, garlic and salt and pepper to taste. Add the oil, a little at a time, whisking until combined well. Stir in the herbs. Pour dressing over mushrooms, turning to coat, and marinate for 15 minutes. Heat grill pan until hot. Add mushrooms and grill for 3 minutes on each side, basting with marinade, or until just firm.