Preheat the oven to 375°F (190°C). Line a sheet pan with parchment paper or aluminum foil.
Tear the bread into pieces and place in the bowl of a food processor. Pulse until coarse crumbs form. You should have 4 to 4 & 1/2 cups of crumbs.
Spread the breadcrumbs in a single layer on the prepared pan. Bake for 5 minutes or until toasted.
Place the toasted breadcrumbs in a bowl. Add the melted butter, sugar, cinnamon, and nutmeg, and stir to combine. Set aside.
To make the filling and assemble:
Toss the apples with the lemon juice. Stir in the brown sugar, cinnamon, and salt.
Spread half of the apple mixture evenly in the bottom of a 3-quart capacity baking pan. Top with half of the breadcrumb mixture. Repeat the layers.
Cover the pan with foil, and bake for 40 minutes. Then remove the foil, and bake 10 to 15 minutes longer, or until the topping is browned and the apples are tender. (If necessary, cover again with foil if the top is getting too brown.)
Notes
Store in an airtight container in the refrigerator up to 3 days.