A simple recipe for mozzarella-stuffed portobello mushrooms comes from Food Network.com. It features portobello mushrooms tossed with balsamic vinegar and olive oil and stuffed with a mixture of mozzarella and Parmesan cheese.
To prepare the mushrooms, preheat the oven to 425 degrees Fahrenheit. Remove the stems and gills from 12 portobello mushrooms. Coat the mushrooms with 6 tablespoons of balsamic vinegar, 1/4 cup of olive oil, gray sea salt and black pepper to taste. Place the mushrooms on a baking sheet, and roast for approximately 10 minutes. Allow the mushrooms to cool to room temperature.
Meanwhile, combine 1 cup bread crumbs, 1 cup grated Parmesan cheese and 4 tablespoons freshly chopped parsley leaves in a mixing bowl. Add 4 tablespoons freshly chopped basil leaves and 1/4 cup olive oil to the bowl. Set aside.
Slice fresh mozzarella into 12 1/2-inch slices, and place one slice in the cupped side of each mushroom. Use two slices if the mushrooms are large. Layer the bread-crumb-and-herb mixture evenly over the mushrooms. Bake the stuffed mushrooms for 5 to 6 minutes, until the mozzarella has melted and the topping is golden brown.
Serve the mushrooms warm, or allow them to cool to room temperature before serving.