Yes, that’s another great thing about this easy party appetizer! Simply make the filling, prep the mushrooms, and then store the mushrooms in an airtight container for up to 3 days before cooking and serving them. I do not recommend freezing the prepped but uncooked mushrooms.  It is possible to prep the filling and freeze it for up to one month. Thaw it out when you’re ready to make the mushrooms, chop up the mushroom stems and stir them into the stuffing before going ahead and stuffing the filling into the mushrooms.