Drop the onions into boiling water for 1 minute. Drain, then peel as soon as they are cool enough to handle.
Cover them with cold water with 1 tespoon salt added. Simmer gently until tender. Do not let them boil or they will break apart. You can do all this the day before.
Make a roux with butter and flour. Add the wine and cream.
Cook and stir over low heat until the sauce is thick (the juice in the onions will thin it). Season with the mace, white pepper to taste, and more salt if you like.
Add the onions and gently heat until ready to serve.
It is usually better to serve a sauced vegetable like this in separate dishes.