Preheat the oven to 325°F with a rack in the center position. Grease a 9x13-inch baking dish with butter.
While you cook the sausages and veggies. Add the torn bread to a rimmed baking sheet lined with parchment paper. Bake, stirring halfway, until dried out for about 25 minutes. Transfer to the prepared baking dish and set aside.
1 loaf French bread
Heat a large skillet over medium-high heat. Melt 4 tablespoons of butter, then add the sausages, break them up into smaller chunks with a wooden spoon. Cook for 4-5 minutes. Remove to a bowl.
1 stick unsalted butter, 2 pounds sweet Italian sausage
In the same pan, melt the remaining 4 tablespoons of butter along with the onions and celery, and cook for 5 minutes on medium heat. Add the garlic, salt, and herbs. Cook for an additional 1 minute. Turn off the heat and add the sausage back in the pan and toss to combine.
1 large onion, 4 stalks celery, 4 cloves garlic, ¼ cup flat-leaf parsley, 1 tablespoon fresh sage, 1 teaspoon kosher salt, 1 tablespoon fresh thyme
Add the sausage and onion mixture to the baking dish along with the toasted bread. Toss to evenly combine.
In a medium bowl, whisk together the eggs and heavy cream, then add in the chicken broth and whisk until combined. Add to the bread mixture and toss together.
1 cup heavy cream, 2 large eggs, 2 cups chicken broth
Cover with foil, and bake until set, about 40 minutes. The mixture shouldn’t move when you jiggle the pan. Discard the foil. Return to the oven uncovered and bake until browned and crispy on top, about 20 minutes.
Remove from the oven, let cool for 5 minutes before serving. Top with some extra chopped parsley and enjoy.