Makes 2-4 servings.
In a large saucepan, melt butter over medium heat and sauté onion until it is translucent. Incorporate flour and mix to form a paste. Cook 5 minutes, stirring constantly. Gradually whisk in cream until smooth. Stir in havarti and smoked cheddar; allow to melt. Fold in green onion.
Place cod fillets in a baking dish and top with sauce. Cover with the crumbled chips. Bake at 350°F (180°C) for 25 minutes.