Boil 200g of pasta in a large saucepan of water until tender, adding 250g of frozen broccoli for the final 3 minutes.
Whisk 40g butter, 40g flour, and 600ml milk in a saucepan over medium heat until bubbling and smooth.
Cook the sauce for 3 minutes until thick, then stir in half the cheese and season with salt and pepper.
Preheat the grill to its highest setting.
Drain the pasta and broccoli, transfer to an ovenproof dish, and flake the tuna on top.
Pour the cheese sauce over the mixture, add sliced tomatoes if using, and sprinkle with remaining cheese.
Grill for 5 minutes until the cheese is melted and golden brown, then serve.