In a saucepan, melt half the butter and brown the onion and the garlic.
Add the mushroom and cook through, until you smell that lovely 'cooked mushroom' meaty smell.
Pour the cream, stirring vigorously, then add the water.
Stirring all the while, add the cheese, 1/4c at a time, Season with the salt as you stir.
Allow to come to a boil, as you stir. When it is boiling, add the flour slurry while stirring quickly. Keep stirring as the flour thickens.
Add the other half of the butter, stirring until it melts through. Taste and season the sauce to your liking.
Serve with steak and chips/fries, or have as a dipping sauce with chips. Refrigerate leftovers in portions/freeze for later eating.