Position a rack in the middle of the oven and
Pour the melted butter into an
In a large bowl, give the eggs a good whisk. Add the cream and whisk again. Add the salt, mustard, sage, pepper, and red pepper flakes, if using, and whisk until combined. Pour the egg mixture over the cheese, making sure it fills in all areas of the dish evenly.
Cut the casserole into squares and serve warm. (Leave the oven on if making sandwiches.)
To make biscuit sandwiches, fill each biscuit with a square of egg casserole and a slice of bacon, if you’re feeling greedy. Wrap each one in foil and bake for 10 minutes.