What's the secret to keeping things warm (like Thomas Keller's Leek Bread Pudding) without continuing to cook/overcook them?

One solution I've seen, and may try myself this year is using a cooler to keep things warm wrapped in towels.

Not sure if it would be ideal for bread pudding, but in an effort to avoid the bottleneck of using the oven for every single thing, I plan to make mashed sweet potatoes ahead of time (stored in a gallon ziploc) and reheat them (in the bag) in a pot of hot (not boiling) water.

I'm planning to make that, too!!! It's SO good.

I'd recommend keeping the bread pudding warm in a low temperature oven, covered with some aluminum foil. The foil should help keep the bread moist and if the oven is around 250° F, you don't have to worry about overcooking! You could also take off the foil just before serving and crisp up the top in a hot (400° F or so!) oven for a delicious top layer.