Giant Stuffed Mushrooms | Sara Moulton

Name: Giant Stuffed Mushrooms | Sara Moulton
Cuisine: Mediterranean
Ingredients: Parsley Pesto (recipe below) or 1 cup store-bought pesto; 1 small onion; 2 tablespoons extra virgin olive oil; 1/2 pound green beans; 6 portobello mushrooms (4- to 5-ounces each); 1 plain English muffin, finely chopped; Kosher salt and freshly ground black pepper; 6 slices Muenster cheese (about 6 ounces)