Sauté the onion in the butter until soft and translucent. Set aside.
Crumble and cook the sausage until done. Set aside.
Combine boxes of stuffing mix, sausage, sage and onion in a large, crockpot cooking bag. Gradually add broth, stirring well with each addition.
Seal the bag and put it in the crock pot on high for 2-3 hours or until steaming and cooked through.
Remove the bag from the crock pot and let it cool slightly before opening. A note on cooking bag: I get the bags designed for crockpots. The instructions call for you to overlap the bag along the edge of the pot, but we loosely tie it closed. The choice is yours.
It should be ready to serve.