INSTRUCTIONS
1. Preheat 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
2. Heat oil in a large skillet over medium heat. Add sausage and cook, breaking up into smaller 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 Tbsp. 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.
3. Preheat oven to 350°F. Butter a 3-qt. shallow baking dish. Whisk eggs to blend in a large bowel; 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 Tbsp butter.
4. Bake until stuffing is hot and the top is golden brown, about 40 minutes.
5.
