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Preheat the oven to 190C/gas 5/fan 170C and get out an ovenproof dish (measuring 20 by 30cm, and about 5cm deep). Meanwhile, put a large pan of water on to boil for the pasta. When it is boiling rapidly, tip in the pasta with a generous sprinkling of salt. Give it a stir, return to the boil and cook for 6 minutes. Add the broccoli, then return the water to the boil and cook for 4 minutes more, until the broccoli is on the firm side of just tender. Drain well.
While the pasta is cooking, put the butter, flour and milk in a large pan and heat, whisking or stirring continuously, until it thickens to make a smooth sauce. Remove from the heat and stir in the mascarpone, sun-dried tomatoes, capers (if using), anchovies (if using) and basil, then add the pasta and broccoli and season well.
Halve the salmon fillets widthways (you will see that there is an obvious divide on each fillet) then place the pieces in a single layer on the base of the ovenproof dish. Spoon the broccoli mixture on top, then scatter with the grated cheddar. (You can chill this for up to 4 hours if you want to get ahead.)
Bake for 30 minutes until the mixture is just starting to bubble round the edges and the mixture is pale golden – don’t let it go too dark or the fish will overcook.