Place a rack in the upper third of the oven; preheat to 300 degrees . Scatter the bread on a rimmed baking sheet. Bake, stirring once, until toasted, about 25 minutes; let cool. Increase the oven temperature to 375 degrees .
In a large skillet, melt the butter over medium-high heat. Add the leeks, celery, bay leaves and rosemary. Cook, stirring often, until the vegetables are soft, about 5 minutes. Add the apples and cook for 1 minute. Remove from the heat. Discard bay leaves.
In a large bowl, whisk together the broth, eggs, 1 1/2 tsp. salt and 1 tsp. pepper. Add the bread and the celery mixture. Toss to combine.
Transfer the stuffing to a greased shallow 3-qt. baking dish; bake until the top is browned, about 45 minutes.