Butter a 9x13-inch baking dish or any of similar size and depth (like a large, oblong Dutch oven).
Whisk together remaining ingredients in a large mixing bowl.
Cut the bread loaf into 1-inch slices.
Dunk each slice into the liquid custard mixture, hydrating each side fully. Layer them, overlapping slightly, into the buttered dish.
Evenly pour remaining custard over the slices. There will be a shallow moat of liquid at the bottom of the pan. Most of that should be soaked up by the bread as it sits, but it can vary depending on the bread.
Let the bread soak, covered, in the fridge over night.
The next morning, or at least a few hours later, remove the pan from the fridge. If there's still a lot of liquid left in the pan, you can drain some of it out.
Preheat oven to 325° F and place a rack in the center position.
Bake for 45-55 minutes, or until the bread is puffed up, there is no more liquid visible in the dish and the tops of the bread are browned and crisp.
Cool for 10 minutes before serving.