Preheat the oven to 200°C/400°F/gas 6. Boil the kettle. Scrub the potatoes, chop into 3cm chunks, place in a large shallow casserole pan and cover with boiling kettle water. Bring back to the boil and cook for 15 minutes, or until tender, trimming and adding the broccoli for the last 4 minutes. Drain, put the broccoli aside, return the spuds to the pan and mash well with the tartare sauce and ½ a tablespoon of olive oil, then season to perfection. Put back on a medium heat and use the back of a spoon to spread the mash across the base and sides of the pan. Drizzle with oil, then transfer to the oven to bake for 20 minutes, or until golden.
Meanwhile, slice the salmon lengthways just under 1cm thick. Halve the cherry tomatoes. Finely chop the chives. Toss it all with the prawns, lemon juice and a pinch of sea salt and black pepper. When the mash is golden, arrange the blanched broccoli on top, then lay over the dressed salmon, prawns and tomatoes. Return to the oven for 10 minutes, or until the fish and prawns are cooked through.