Place the bread in a large bowl. Grease a 9 by 13-inch casserole dish with the remaining tablespoon of butter and set aside.
Combine the heavy cream, milk, eggs, brown sugar, vanilla, cinnamon, nutmeg, salt, and raisins in a large bowl. Whisk to mix. Pour the cream mixture over the bread, and stir to combine. Allow the mixture to sit at room temperature for 30 to 45 minutes.