Method:
Whisk together oil, vinegar, rosemary, parsley, Italian herbs, pepper flakes, salt and pepper in a large bowl.
Arrange mushrooms in a wide, shallow dish and pour about 3 tablespoons of the vinaigrette over the top. Turn to coat all over and set aside to let marinate at room temperature for 20 minutes. Grill mushrooms over hot coals or on a gas grill over medium heat for 5 minutes per side, until nicely browned and tender. (Or, broil the mushrooms 3 inches from the broiling element of your oven for 6 to 7 minutes per side, until lightly browned.) Cut mushrooms into 1-inch thick slices.
Arrange cheese on one piece of the bread, then top with mushrooms. Top with peppers and arugula. If you like, sprinkle a little of the remaining vinaigrette over the arugula, then top with remaining piece of bread. Cut sandwich in half, transfer to 2 plates and serve.