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 cream mixture in a slow, steady stream into egg mixture. Pour over bread; fold to combine. Let stand 30 minutes, tossing and pressing occasionally to submerge bread.
Meanwhile, soak raisins in 1 cup boiling water for 30 minutes.
Drain; stir raisins into bread mixture. Preheat oven to 350 degrees. With a slotted spoon, transfer bread to buttered dish; pour liquid in bowl over top. Using spoon, turn top layer of bread crust side up.
Set dish in a roasting pan; transfer to oven. Pour boiling water into pan to reach about halfway up sides of dish. Bake until golden brown, about 50 minutes. Let dish cool on a rack 10 to 20 minutes.