Bake the bread (if not using store -bought) and cornbread two days ahead.
The day before you want to make the stuffing, crumble the cornbread and mix with the bread chunks in a large bowl. Cover with a kitchen towel, so it can dry out a little.
Combine broth, onions, celery, nutritional yeast, sage, mustard powder, and black pepper in a medium saucepan.
Bring to a boil, then simmer for 5 minutes.
Remove broth mixture from heat.
Remove broth mixture from heat and stir into bread mixture, until very moist, but not soggy. Add salt and adjust seasonings as necessary.
Preheat oven to 350 degrees F.
Spoon into a baking dish and bake for 30 minutes, or until heated through.