Preheat oven to 200°C fan. Boil pasta 2 minutes less than packet directs. Drain.
Melt butter in a heavy saucepan. Whisk in flour. Cook 90 seconds.
Slowly whisk in milk. Bring to gentle simmer. Cook 5 minutes whisking until thickened.
Off heat, stir in 200g cheddar, all the gruyere, 70g parmesan, mustard and nutmeg.
Combine pasta and sauce. Pour into a deep baking dish. Top with remaining cheddar and parmesan.
Combine panko, melted butter and paprika. Sprinkle over the top.
Bake 30-35 minutes until deeply golden and bubbling at the edges. Rest 10 minutes before serving.