Heat ghee, garlic and onion in a small saucepan until the garlic starts to caramelize and the onions are softened, about 10 minutes. Allow ghee to cool to room temperature.
Place mushrooms, ghee mixture and a pinch of salt in a vacuum-seal bag (or heavy duty Ziploc bag.)
If you have a sous vide machine, set your temperature to 185° F. If you don’t, bring a large pot of water up to 185° F. Monitor the temperature with a cooking/candy thermometer.
Once the water is up to temperature, place the sealed mushrooms into the water bath. Cook for 45 minutes to 1 hour.
While the mushrooms are cooking, prepare your mousse.