Crab Tuna Souffle

In a small bowl, stir together the crabmeat and tuna. Place a layer of bread cubes in the bottom of a 1 1/2 quart casserole dish. Cover with a layer of the crab and tuna, then a layer of cheese and a sprinkle of parsley. Repeat layers 2 more times ending with cheese and parsley on top. In a medium bowl, whisk together the eggs, milk, butter, mustard powder and onion. Pour over the layers in the dish. Cover and refrigerate for at least 3 hours, or overnight. Preheat the oven to 350 degrees F (175 degrees C). Let the dish come to room temperature while the oven preheats. Bake uncovered for 1 hour and 15 minutes in the preheated oven, or until puffed and golden.