1. Cook onions in boiling water until tender. Allow to cool, peel and set aside.
2. In a saucepan, melt butter over medium low heat. Add flour and stir.
3. Pour in the chicken broth and heavy cream and simmer, stirring constantly, until thick.
4. Remove from heat and stir in the parsley and parmesan cheese.
5. Add the onions and stir to coat.
6. Transfer the onion mixture to a baking dish and cover with plastic wrap. Store in the refrigerator overnight.
7. When ready to cook, remove the pan from the refrigerator and allow to rest at room temperature for 30 minutes.
8. Preheat the oven to 350 degrees. Bake the onions covered for 15 minutes, then remove the cover and cook an additional 10 minutes, until the sauce is bubbly.