Preheat the grill to its highest setting.
Melt 25g of butter and whisk in the flour to form a roux.
Gradually whisk the warm milk and cream into the roux until thickened and set aside.
Fry the cooked haddock in the remaining butter until heated through.
Pour the seasoned eggs over the fish and stir in the prawns until the eggs are lightly set.
Mix 50g of parmesan and the egg yolk into the roux sauce and season.
Spread the sauce over the omelette and sprinkle with the remaining parmesan.
Grill for 30 seconds until the cheese is melted and blistering.
Slide the omelette onto a plate and cut it into slices to serve.