Arrange bread on rimmed baking sheet; toast in 350°F (180°C) oven until light golden, 10 to 12 minutes. Transfer to large bowl.

While bread is toasting, in large skillet, melt half of the butter over medium-high heat; cook sausage, breaking up with spoon, until browned, about 8 minutes. Using slotted spoon, remove to bowl with bread.

In same skillet, melt remaining butter over medium heat; cook leeks and celery, stirring occasionally, until beginning to soften, about 6 minutes. Add apples, pepper and salt; cook, stirring, until tender-crisp, about 4 minutes. Stir into bread mixture. Add parsley and sage.

In bowl, whisk broth with eggs; drizzle over bread mixture. Toss to coat.

Scrape into greased 13- x 9-inch (3 L) baking dish; cover with foil. (Make-ahead: Refrigerate for up to 24 hours. Let stand at room temperature for 1 hour before continuing with recipe.) Bake in 425°F (220°C) oven for 20 minutes; uncover and bake until top is browned, 10 to 15 minutes.