For the casserole: Generously butter a 3-quart casserole dish. Arrange the bread slices over the bottom of the dish in three rows of 6 slices, shingling slightly.
Whisk the eggs, granulated sugar, vanilla, cinnamon, nutmeg and 1/2 teaspoon salt in large bowl until combined and smooth. Whisk in the milk and heavy cream. Pour the egg mixture evenly over the bread slices, making sure that all slices have been coated. Cover the dish and refrigerate for at least 8 hours or up to 12.
Position an oven rack in the center of the oven, and preheat to 350 degrees F.
For the crumble: Toss the brown sugar, flour, cinnamon, nutmeg and 1/4 teaspoon salt together in a medium bowl. Add the butter and work it in with your fingers until the mixture is crumbly with pea-sized bits of butter. Mix in the walnuts.
To assemble: Give the bread slices in the casserole dish a gentle push into the custard. Sprinkle the crumble evenly over the top.
Bake the casserole until the top is puffed and browned and a knife inserted in the center comes out clean, about 45 minutes (it will still be a little wiggly but will continue to cook a bit out of the oven); check halfway through the baking time and tent with foil if the topping is browning too quickly. Serve hot.
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