Cut bread into ½ inch cubes and spread it out on a large baking sheet. Leave it out to get stale for about a day.
Cooking mushrooms:
Preheat the oven to 350° and grease a 9x13 casserole baking dish with butter all over the bottom and up the sides.
Preheat a large cooking pan over medium heat and add oil. Sauté shallots and leeks until softened and start to brown and then add pressed garlic. Sauté just until garlic is fragrant.
Add sliced mushrooms and season them with a little bit of salt. (Not too much because egg mixture already has salt.)
Cook mushrooms, stirring from time to time, until completely cooked. Add butter, let it melt, and stir it in with mushrooms.
Take the pan off heat, make a little well in the center of mushrooms and add sherry. Place the pan back on the stove and let it simmer for a minute or so. Stir mushrooms to coat and take off heat.
White eggs, heavy whipping cream, stock, and seasoning together and set aside.
Cooking bread pudding:
Spread stale bread in the prepared baking dish evenly.
Spread cooked mushroom mixture over the bread and nestle some in between bread cubes.
Spread grated cheddar cheese over the top and evenly pour the egg mixture all over.
Gently press on the bread to help it soak up the mixture. Let everything sit on the counter about 10-15 minute before baking.
Bake bread pudding for 40-45 minutes. (If the top is browning too fast, you can cover the top loosely with a sheet of aluminum foil half way through or so.)
Notes
How To Tell When Bread Pudding Is Done: You can always touch the middle of the pudding and see how jiggly it is. If the middle hasn't baked through yet, it will be quite jiggly and it even might feel a little squishy. If it starts to feel firm and yields when you gently press into it, it is fully cooked. I also like to check the internal temperature in the center of bread pudding and aim for 170°F-175°F.Need To Make Less? If you don’t need that much bread pudding, you can easily half the recipe and use a 2-2.5 quart baking dish, or even an 8×8 dish. The baking time won’t be affected because you are using a smaller dish, since the thickness of the bread pudding is still the same.Storing: you can easily store bread pudding in the same dish you baked it in. Make sure to cover it air-tight with plastic wrap or use a baking dish that comes with a lid. Store it for 3-4 days in the refrigerator! To reheat, simply cut off a slice and microwave just until heated through.