Slice the mushroooms thinly, and fry lightly in some butter or a bit of oil. (You could use tinned, drained mushrooms).
Melt butter in a small saucepan, and off the heat stir in the flour briskly with the back of a spoon until mixture is smooth.
Keep off heat and mix in the wine, garlic, sugar, white pepper and salt.
Return to the hob, and whisk over medium heat until it reaches boiling point and thickens. Simmer about 30 seconds, then take off heat.
Drain any liquid off the mushrooms, and add mushrooms to the sauce.
Stir to cool a little, and whisk in cream. Add parsley last.
(It's a good idea to prepare the rice and seafood first, as this sauce can't wait too long in a warming oven. Put the rice on a heated platter, arrange seafood or fish on top, and pour over the sauce).
*You can obviously use black pepper, but white pepper has almost been forgotten and can be quite nice in this sauce.