Heat the butter in a large skillet until it has just begun to brown. Add the mushrooms and stir. Cover and cook on medium heat for a good 10 minutes, stirring every so often, until mushrooms have shrunk in size and have wilted. Add two turns of the pepper grinder to the pan. This will take longer than you think it should – but don’t rush the mushrooms!
Add the onion to the mushrooms, along with a few more turns of the pepper grinder and a light sprinkling of kosher salt. Stir. Cook on medium-high heat until the onions are translucent. Drop in the cream cheese and chicken stock. Cover the skillet again and cook on medium heat until the cheese has melted fully. Turn off the stove.
Put the trimmed and rinsed green beans into an empty 6-quart slow cooker insert and pour the mushroom mixture evenly over the top. Stir to disperse the ingredients. Cover and cook on low for 5 hours, or on high for about 2 hours. Stir again, and serve as-is, or top with a cup of fried onions.