Preheat an oven to 350°F and position a rack in the center. Generously butter a large casserole or baking dish.

Cut off crust from bottom and short ends of bread and discard. Cut bread into ¾-inch cubes. Place bread cubes into a large bowl and toss with oil and garlic. Evenly arrange bread cubes onto a rimmed baking sheet. Transfer to the oven to bake until golden and slightly dry, tossing occasionally, about 15 minutes. Cool slightly and transfer back to the large bowl.

Place a heavy deep skillet on the stove over medium-high heat, add the sausage and cook until cooked through, crumbling coarsely with the back of a spoon, about 10 minutes. Using a slotted spoon, transfer sausage to the bowl with bread cubes and set aside.

Return the skillet to medium heat, add butter. When foam subsides, add leeks, apple and celery and cook until tender, about 5 minutes. Stir in dried cranberries, rosemary and parsley. Transfer mixture to the bowl with sausage and bread. Season to taste with salt and pepper. (Recipe can be prepared up to this stage 1 day ahead. Cover and refrigerate).

Using a silicone spatula, stir eggs and broth into the mixture until thoroughly combined. Transfer to the prepared baking dish. Cover with foil and bake until heated through and almost set, about 45 minutes. Remove the foil and bake until golden, about 15 minutes longer.