- 1 loaf brioche or challah (about 12 ounces), cut into 1″ cubes
- 1 tablespoon extra-virgin olive oil
- 1 pound hot Italian sausage, casing removed
- 4 tablespoons unsalted butter, divided, plus more for dish
- 1 yellow onion, chopped (about 1 1/2 cups)
- 1 medium fennel bulb, cored and coarsely chopped (about 3 cups)
- 4 large eggs, beaten
- 2 1/2 cups low-salt chicken broth
- 2 pounds frozen spinach, thawed, well squeezed, coarsely chopped
- 2 teaspoons kosher salt
- 1 1/2 teaspoons freshly ground black pepper
- 1 1/2 teaspoons fennel seeds
- Preheat oven to 350°F. Scatter bread on a rimmed baking sheet. Toast until golden brown, tossing once, about 15 minutes. Let cool completely. DO AHEAD: Bread can be toasted 1 day ahead. Store airtight at room temperature.
- Heat oil in a large skillet over medium heat. Add sausage and cook, breaking up into small pieces with the back of a spoon, until browned and cooked through, 8-10 minutes. Drain on a paper towel-lined plate; let cool. Melt 2 tablespoons butter in the same skillet over medium-low heat. Add onion and fennel and cook, stirring occasionally, until soft, 10-12 minutes.DO AHEAD: Can be made 1 day ahead. Let cool; cover and chill.
- Preheat oven to 350°F. Butter a 3-quart shallow baking dish. Whisk eggs to blend in a large bowl; whisk in broth. Stir in sausage, the onion-fennel mixture, spinach, salt, pepper, and fennel seeds. Add bread; toss until evenly distributed and bread has absorbed the liquid. Transfer to prepared dish; dot with remaining 2 tablespoons butter.
- Bake until stuffing is hot and the top is golden brown, about 40 minutes.