Heat the oven to 350°.
Heat butter in 3-quart saucepan over medium heat.
Add onions and cook for 15 minutes or until well browned, stirring occasionally.
Stir pancetta, garlic, celery and rosemary in saucepan and cook until pancetta is well browned, stirring occasionally.
Stir in broth and sherry and heat to boil.
Remove saucepan from heat.
Add stuffing and mix lightly.
Spoon stuffing mixture into greased 3-quart casserole.
Bake for 30 minutes or until stuffing mixture is hot.