Butter a 9-by-13-inch baking dish; set aside. Put bread in a large bowl; set aside.
Heat milk and cream in a medium saucepan over medium-high heat until just about to simmer; remove from heat.
Whisk eggs, yolk, sugar, salt, vanilla, cinnamon, and nutmeg in a medium bowl.
Whisking constantly, pour the cream mixture in a slow, steady stream into the egg mixture.
Pour over bread; fold to combine. Let stand for 30 minutes, tossing and pressing occasionally to submerge the bread.
Meanwhile, soak raisins in 1 cup boiling water for 30 minutes.
Preheat oven to 350°F. Drain raisins then stir into bread mixture.
Use a slotted spoon to transfer bread to the buttered dish; pour liquid in a bowl over top. Using a spoon, turn the top layer of the bread crust side up.
Set dish in a roasting pan; transfer to oven. Pour boiling water into the pan to reach about halfway up the sides of the dish.
Bake until golden brown, about 50 minutes. Let dish cool on a rack 10 to 20 minutes.