Heat the butter in a 3-quart saucepan over medium heat.
Add the onions and cook for 15 minutes or until well browned, stirring occasionally.
Stir the pancetta, garlic, celery and rosemary in the saucepan and cook until the pancetta is well browned, stirring occasionally. Stir in the broth and sherry and heat to a boil.
Remove the saucepan from the heat.
Add the stuffing and mix lightly. Spoon the stuffing mixture into a greased 3-quart casserole. Cover the casserole.
Bake for 30 minutes or until the stuffing mixture is hot.