Melt butter in sauce pan. Saute onions until tender. Be careful not to burn butter.
In another bowl (I use a Pyrex 4-cup measuring cup), warm milk in microwave for 1 minute.
Add flour to butter & onions and cook, stirring constantly, for 1 minute.
Add warmed milk and broth to saucepan gradually, while continuing to stir.
Cube cream cheese and add to saucepan.
When cream cheese starts to melt, add green bean casserole.
Season to taste with salt, pepper & nutmeg.
Continue heating and stirring until soup thickens. Do not boil.
Garnish with French's onions if you have them leftover from making your casserole too.