Look for firm, evenly colored mushrooms with closed caps. If all the gills are showing, then the mushrooms are past their prime. Fresh mushrooms should be wiped with a damp paper towel or rinsed in cold water and dried thoroughly before using. Mushrooms should never be soaked because they absorb water and will become mushy.
Try to buy mushrooms that are about the same size so that they cook evenly in this recipe.
This dish can be made up to 2 days ahead and refrigerated until ready to serve.