Clean mushrooms thoroughly, then place in bottom of slow cooker.
Whisk ranch dressing mix into melted butter, then pour over mushrooms. Stir to coat.
Cover and cook on LOW for 3 hours. Check for desired tenderness, and cook a little longer if needed.
If you're feeding more than 2-3 mushroom-loving people, I suggest doubling the recipe:
2 lbs fresh mushrooms
3/4 cup butter (1.5 sticks)
Drizzle of extra virgin olive oil
2 packets of dry ranch dressing mix
Cook as directed.