Preheat oven to 400 degrees F. Place bread cubes in two 15x10x1-inch baking pans or shallow roasting pans. Bake 15 to 20 minutes or until crisp, rotating and stirring once. Set aside.
Heat a very large skillet over medium-high heat. Add bacon and cook until crisp; drain bacon on paper towels, reserving 3 tablespoons drippings in skillet. Crumble bacon; set aside. Add mushrooms to skillet; cook about 7 minutes or until just brown. Stir in leeks and garlic; cook until just tender. Add apples; cook for 1 to 2 minutes or until apples begin to soften. Remove from heat; add crumbled bacon and set aside to cool for 10 minutes.
In a 4-quart casserole or Dutch oven combine eggs, milk, cheese, salt, pepper, bread cubes, and the leek mixture; toss well. Cover and refrigerate 3 hours or overnight.
Remove from refrigerator. Let stand 30 minutes. Preheat oven to 400 degrees F. Bake, covered, for 45 minutes. Uncover and bake 30 minutes more or until an instant-read thermometer inserted into center registers 170 degrees F. Let stand 20 minutes before serving. Makes 12 servings.