Preheat the oven to 375°F. Butter the bottom of a 9×13-inch baking pan with 1 tablespoon butter.
Cook the Sausage
Heat a large skillet over medium heat. Cook 1 pound mild Italian sausage, breaking it up with a spoon, until no pink remains, about 6 to 8 minutes. Transfer to a bowl.
Sauté Vegetables
Add remaining 11 tablespoons butter to the skillet. Stir in 2 cups diced onion and 1½ cups diced celery. Sauté until vegetables are translucent, about 5 to 7 minutes.
Mix in 2 tablespoons chopped parsley, 1 tablespoon chopped sage, 1 tablespoon chopped thyme, 2 teaspoons poultry seasoning, 1 teaspoon kosher salt and ½ teaspoon pepper. Cook for 1 minute until fragrant.
Assemble & Bake
In a large bowl, combine the sausage and vegetable mixture with 12 cups cubed bread.
Whisk 3 beaten eggs and 3 cups chicken stock together. Pour over the bread mixture and toss gently until evenly moistened.
Transfer to the prepared pan and cover with foil. Bake for 25 minutes. Remove foil and bake an additional 15 minutes until the top is golden and crispy around the edges.
Notes
Make sure to dry the bread cubes for at least 4 hours so they absorb broth evenly.You can prepare up to whisking eggs and stock the night before; cover and refrigerate, then bake when ready.Leftovers keep in an airtight container for up to 4 days refrigerated or 2 months frozen.