Preheat the oven to 350 degrees.
In a large bowl, whisk the milk, cream, mustard, and eggs until thoroughly mixed. Season the mixture generously with salt and pepper. Add the bread cubes to the egg mixture and toss to combine, making sure every piece is saturated. Set aside.
Meanwhile, in a large Dutch oven, cook the sausage over medium-high heat until cooked through. Remove the sausage from the pot and reserve 2 tablespoons of the pan drippings. Heat the reserved drippings and butter; add the onions and celery and saute until soft and translucent, about 5 minutes.
Add the greens to the Dutch oven and cover. Cook for 2 minutes, remove the lid, and season generously with salt and pepper. Continue to cook uncovered, stirring occasionally, for an additional 10 minutes. Remove from the heat and cool for a few minutes.
Grease a 9 x 13 casserole dish. Toss the greens and sausage with the bread mixture until well combined. Pour the dressing into the dish. Allow to sit for approximately 45 minutes to 1 hour. Cover with aluminum foil and bake for 20 minutes. Remove foil and cook for an additional 25 minutes, or until the top is crisp and golden brown.