Preheat oven to 250°F. Tear pretzels into bite-sized chunks, then slice them in half down the middle (like how you'd slice a bagel). Tear the pretzel buns into 1-2" cubes. Toast in an even layer on an ungreased sheet pan for ~1 hour, tossing every 15 minutes. Remove from the oven when dried and place in a very large bowl.
Increase oven temperature to 375°F.Melt butter in a medium pan over medium heat. Add the ground sausage and use a spoon to break it into crumbles while it cooks. When the sausage is mostly cooked through and broken into small pieces, add the carrots, onions, and celery. Sauté until onions are translucent and golden, but not brown. Add mixture to the bowl with the pretzel pieces along with parsley, rosemary, thyme, sage, salt, and pepper. Mix and stir to combine.
Add ½ cup of chicken stock and 1 TBSP whole grain mustard to the pretzel mixture. If you’re adding msg, sprinkle it in now too. Stir to combine.
In a small bowl beat together ¾ cup stock and 1 large egg. Add to pretzel mixture and stir again to thoroughly combine and evenly distribute all the ingredients.
Let mixture sit for 15-20 minutes. Check the pretzel texture — if they’re still quite hard, add another ¼ cup chicken stock and mix again.
Transfer the mixture to a well-greased or well-buttered casserole dish and sprinkle pretzel salt over the top. Cover loosely with aluminum foil.
Bake for 20 minutes at 375°F, then remove the foil, rotate the dish, and reduce oven temp to 350°F. Bake for another 20 minutes.
When the stuffing is deeply golden brown and crunchy on top it’s done. Take it out of the oven and let it rest for 10 mins before serving.