Add fat to the pan, wait until it is hot and fry onions on medium heat until translucent. Don't forget to add the salt and stir!
Add bacon, stir the whole thing again and keep frying for another 2 minutes.
Add mushrooms and wait until soft.
Add broth and wait until it starts cooking. Add cream, wait until the ingredients combine and cook again. When you see the bubbles you should turn the heat to medium low and let it simmer for up to 15 minutes.
Add beurre manié and stir the sauce. After 1-2 minutes you should see how thick the sauce got :)
Season with salt, pepper and fresh parsley. If o you want to reheat the sauce just add a bit of milk before.