My mom used to make these for us

While the pan is heated over medium heat, melt the butter.
The flour should be whisked in to make a roux. Remove from heat and let cool for two or three minutes, or until browned.
Make sure there are no lumps by whisking in the heavy cream and milk gradually.
The sauce will thicken as it simmers, so be sure to stir it often.

Cook for a while:

Mix in the nutmeg, salt, and white pepper to taste along with the sauce.
Simmer, covered, for 15 to 20 minutes, or until pearl onions are soft, after peeling them.
Get it and enjoy it!

Take the saucepan off the heat when the onions are soft and the sauce thickens to your liking.
Before serving, top with chopped fresh parsley.