Melt the butter on low heat.
Add the garlic and sauté for a minute - until the fragrance turns from spicy to sweet.
When it stops bubbling add the flour and stir. Brown the flour a bit.
Slowly add the heavy cream while stirring to avoid creating clumps.
Add the blue cheese nutmeg, salt, and pepper and let simmer on low heat until the cheese melts, stir occasionally to avoid the sauce getting burned.
Check the sauce consistency, if it is too thick to your liking add a bit of milk. if it is too thin add the corn starch - combine the corn starch with room temperature water in a cup and add slowly while stirring to avoid creating clumps.
If adding mushrooms, sauté them in butter in a separate pan until they lose their liquid, then add them to the sauce.
Serve with grated parmesan on top