Making a casserole for Christmas morning works so well because you can assemble it the night before, pop it in the fridge and in the morning, all you have to do is put it in the oven!
Technique tip: Allowing this to set overnight ensures the custard soaks into the bread perfectly, and the interior develops a soufflé like texture as it bakes.
Swap option: Replace the bread with any you have on hand, same with the sausage, Italian sausage works great here too! And if you don't have leeks, use a large yellow onion instead.
In a large skillet over medium heat, add the olive oil; once hot, add the breakfast sausage, break it up as much as you can with a wooden spoon, and once it's halfway cooked, add the leeks, along with a pinch of salt and pepper and allow to cook for about 10 minutes until the leeks have softened and slightly caramelized (you might have to increase the heat a bit after a few minutes). Once cooked, allow to cool slightly.2.
In a large bowl, whisk together the eggs, milk, mustard, salt and pepper. Set aside.3.
Add the cooked sausage mixture, bread and 1 cup of the cheese and parsley to the bowl and give everything a good stir for few minutes, allowing the bread to soak up as much of the custard as possible.4.
Pour the mixture into a greased 9- by 13-inch baking pan (preferably glass or ceramic) top with remaining cheese, cover with plastic wrap and pop it in the fridge overnight.5.
Preheat oven to 350°F.6.
Allow the casserole to come to room temperature for about half an hour, then bake for an hour or until puffed and golden.