Add cheddar and Parmesan cheeses. Beat in egg yolks, mustard and seasonings.
Beat egg whites in a bowl until they form stiff peaks. Using a spatula, gently fold into warm sauce until evenly incorporated.
Spoon into a soufflé dish and place in a baking pan of hot water. Bake for 15-20 minutes, or until well risen and cooked through. Serve immediately.
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