I feel ya on this I cooked so much with CoM soup in my old life it was one of the first things I figured out how to make!
It's easy but sounds complicated. Tools needed: Cast iron skillet or one that can create blackened drippings. Ingredients: Mushrooms, heavy whipping cream, butter, quality salt and pepper, garlic powder and the my big helper is dry onions.
In your large skillet melt the butter, and fry up mushrroms, The amounts are personal but I use only enough butter to easily sautee the mushrooms. Add the dry onions and spices and you will need to use a metal or strong utensil and be moving things around and scraping pan bottom. After you get a nice coat of drippings and the mushrooms are very cooked. remove the mushrooms or push to one side and turn off the heat. Add another spoon of butter and splash of cream, it will instantly bubble the drippings loose so be stirring and scraping. Turn burner back on. Keep adding heavy cream a little at a time and you should notice that mixture is thick and creamy. Stop when you feel you have enough. If it gets to thin you can add an egg yolk or two by adding some of the hot 'gravy' to the yolks first to temper then adding to the pan. Or if all else fails add 1/2 tsp tapioca flour to some water and pour into pan (this is my least favorite). Do this on a low enough heat that you are in control at all times. and you can turn the heat off and on or remove from burner any time. This method works for all kinds of sauces and thick juicy toppings. Fry some bacon then just add buitter, wine or water to the drippings and get a nice sauce. Or add balsamic, butter and cream! Any liquids works with a nice layer of pan drippings. Good luck..