This is desirable for use in recipes because it makes the mushrooms look better and it also removes any dirt clinging to the surface of the mushroom.
To remove the gills, scrape the underside of the mushroom with a spoon. This is an important step, because the gills are very dark brown and they will destroy the visual appeal of your dish if you leave them on the mushrooms.
I was using portobello mushrooms in a recipe for Hungarian Mushroom Soup that I got in culinary school.
The recipe is as follows:
1/2 c. dried porcini mushrooms (.5 ounces), soaked in warm water
1 cup onion, diced
1 Tb garlic, chopped
4 Tb butter
1 lb portabella mushroom caps, gills removed and diced
1 Tb Hungarian Paprika
4 Tb flour
4 c. chicken stock
1/2 cup parsley, chopped
salt and freshly ground pepper
1 cup sour cream or creme fraiche as garnish
1. Scoop the porcini out of the water and chop. Strain the liquid through a cheese to remove the grit cloth and retain the liquid.
2. In a saucepan, melt the butter and saute the onions in it until translucent and then add the garlic. Continue to cook until softened.
3. Add portobellos and porcini. Season with salt. Saute until the portobellos give off liquid.
4. Stir in the paprika and flour and toast it for a minute over the heat. Pour in half of the stock and simmer until thickened and smooth. Pour in the remaining stock and porcini mushroom liquid. Simmer 20-30 minutes or until it reaches the desired consistency and add chopped parsley. Season with salt and pepper to taste. Serve with a dollop of sour cream or creme fraiche.
People in the culinary world used to think that you should never wash mushrooms before use, because they act as sponges and soak up a lot of water which could then affect the recipe. But now we know this is not true. Also, typical mushrooms used for culinary applications are not grown in manure, so you can put to rest any fears you have about this. See here: The Mushroom Lady