Heat the oven to 375 degrees.
Boil the onions in a pot of water for 6-8 minutes and drain well.
In a separate pot, melt the butter over medium heat.
Stir in the flour, and cook for 1 minute, stirring often.
Slowly whisk the broth and heavy whipping cream in the skillet.
Stir in the nutmeg, ¼ cup of the parmesan cheese and salt and pepper to taste. Simmer for 1-2 minutes.
Add the onions into the sauce mixture.
Spread the mixture into a well greased 9x9 baking dish, and top with the remaining cheese.
Bake for 30-35 minutes.