Place 6 slices of bread in a single layer in the bottom of the baking pan. Layer the Spam or ham over the bread, and cover with the cheddar cheese. Then place the remaining 6 slices of bread in a single layer over the cheese.
In a mixing bowl, whisk together the eggs, milk, and dry mustard. Pour the egg mixture over the strata and refrigerate overnight.
Remove the strata from the refrigerator, sprinkle with corn flakes or potato chips, and bake for 1 hour at 350°F.