1 loaf brioche or challah (about 12 oz.), 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 coarsly chopped (about 3 cups)
4 large eggs, beaten
2 1/2 cups low-salt chicken broth
2 pounds frozen spinach, thawed, well squeezed, coarsly chopped
2 teaspoons kosher salt
1 1/2 teaspoons freshly ground black pepper
1 1/2 teaspoons fennel seeds