Heat the grill to high. Arrange the tomatoes and salmon over a baking tray, season and drizzle with half the oil. Grill for 10 mins until the salmon is cooked through and the tomatoes are juicy. Leave to cool.
Cook the penne following pack instructions, until al dente. Add the broccoli for the final 2 mins of cooking time. Drain the pasta, reserving a large mugful of water.
Return the pasta and broccoli to the pan, then stir in the pesto, mascarpone, half the tomatoes and enough of the reserved pasta water to thin the sauce to the consistency of double cream. Flake in the salmon, discarding the skin. Tip into a large baking dish, then mix the breadcrumbs with the parmesan and remaining oil, and scatter this over the mixture. Dot the rest of the tomatoes on top. If baking straightaway, heat the oven to 200C/180C fan/gas 6. Or, cover and chill until you’re ready to cook. Will keep chilled for up to 24 hours.
Bake for 20 mins (or 25 mins from chilled) until the top is golden and crunchy.