You can’t freeze fresh mushrooms, because their water content is far too high. So when I’ve got some I won’t use this week, I turn them into a purée instead.
Break them into a roasting tin, stalks and all, with a handful of porcini if you have them. Dot with about 50g butter, add thyme and a fat halved garlic clove, cover tightly with foil and roast at 120°C for an hour. Pick out the thyme stalks and blitz everything, juices and all. Season, but add no dairy yet. You can add that after defrosting.
Cool completely, freeze in large ice cube trays, then bag them up. Perfect for risottos, soups and sauces.