Crunchy-melty tuna and broccoli pasta bake

Method

1. Heat the oven to 200°C/180°C fan/gas 6. In a small bowl, combine the chopped onions and vinegar, then set aside (this helps to mellow the flavour of the onion).
2. Bring a large pan of salted water to the boil and cook the pasta for 2 minutes less than the packet instructions. Add the broccoli to the pasta pan 2 minutes before the end of the reduced cooking time. Drain both and return to the pan, drizzle over a little olive oil, then stir and set the pan aside.
3. While the pasta is cooking, make the white sauce. Melt the butter in a large saucepan over a medium heat. Add the flour and stir to make a roux, then cook for 1-2 minutes. Remove from the heat and gradually add the milk, stirring well after each addition to ensure there are no lumps. Return to the heat, bring to the boil, stirring constantly, then simmer for 2 minutes until thickened. Remove from the heat, stir in the dijon mustard, 100g cheddar and 50g gruyère and allow to melt.
4. Drain the chopped onion (discard the vinegar) and add to the white sauce in the pan along with the drained pasta and broccoli, capers, tuna and half the parsley. Transfer to a large ovenproof dish, then scatter over the remaining cheeses. Toss the sourdough chunks in 1 tbsp oil, then sprinkle over the top (see Make Ahead). Bake for 30 minutes until the croutons are golden and the bake is bubbling. Serve scattered with the remaining chopped parsley.