Melt butter in a cast iron pan or a medium sized pot. Add veggies and parsley, salt, pepper, and sage. Sauté on medium heat until vegetables are soft, 10 minutes or so.
While that's cooking, break up a loaf of bread into cubes into a mixing bowl.
When veggies are done, pour all of that over the top of the bread and stir well. Add a beaten egg if desired, for more nutrition & moisture. Add enough broth to get it as moist as you like. We like it moist but not wet.
Stir until it's all mixed in, then transfer to a buttered dish or stuff your bird once it's rinsed out well.
Bake 30-40 minutes on 350*, OR if you're stuffing a bird, it will take much longer, obviously, since it depends on the weight. See specifics below for more on that and how much to make.