Lepiota rachodes is especially well suited for stuffing, for several reasons. It has a mild flavor that complements and blends well with the juices from the stuffing. It grows large enough that the young round caps can hold enough to make a meal. It has a firm texture with low moisture, giving it a good structure for stuffing. Lastly, it rarely has any bugs, so you don't have to feel nervous about serving an entire cap. If you can't find Lepiota, you can substitute any large young agaricus (except perhaps augustus whose almondy flavor might not blend well with the stuffing.) A large Amanita caesara or calyptrata cap will also work, and giant morels would be heavenly. If it's the wrong time of year, you can use the large brown Agaricus bisporus sold in stores as "Portobello."
Clean the mushrooms with a brush or dry sponge, using minimum water. Carefully remove the stems and set the caps aside to be stuffed later. Chop the stems finely and sautée for 5-10 minutes on low heat, without oil, to soften and reduce moisture. Place the cooked stem pieces into a mixing bowl and mash together with the remaining ingredients. Spoon the mixture into the mushroom caps.
Place the filled caps, open side up, onto a lightly oiled cookie sheet. Cook for 35-40 minutes at 300°F in a preheated oven. Remove and sprinkle the chopped parsley on the top. Serve immediately.
Variations: You can bulk up the filling by adding cooked rice to the blend, increasing the herbs accordingly. Chopped tomato will add a pleasant acidity to the flavor, but at the risk of overpowering the mushroom. Chopped black olives will add a savory tannic twist.