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1
For the casserole: Generously butter a 3-quart casserole dish.
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2
Arrange the bread slices over the bottom of the dish in three rows of 6 slices, shingling slightly.
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3
Whisk the eggs, granulated sugar, vanilla, cinnamon, nutmeg and 1/2 teaspoon salt in large bowl until combined and smooth.
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4
Whisk in the milk and heavy cream.
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5
Pour the egg mixture evenly over the bread slices, making sure that all slices have been coated.
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6
Cover the dish and refrigerate for at least 8 hours or up to 12.
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7
Position an oven rack in the center of the oven, and preheat to 350 degrees F.
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8
For the crumble: Toss the brown sugar, flour, cinnamon, nutmeg and 1/4 teaspoon salt together in a medium bowl.
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9
Add the butter and work it in with your fingers until the mixture is crumbly with pea-sized bits of butter.
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10
Mix in the walnuts.
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11
To assemble: Give the bread slices in the casserole dish a gentle push into the custard.
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12
Sprinkle the crumble evenly over the top.
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13
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.
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14
Serve hot.
