Preheat the oven to 180°C/350°F/gas mark 4. Place the salmon onto a baking tray and sprinkle with salt and pepper then drizzle with a little olive oil and place in the hot oven for 8 to 10 minutes.
While the salmon is in the oven, make the Marie Rose dressing. Mix the yoghurt and ketchup in a small bowl then add the zest and juice of one of the lemons. Season well with salt and pepper and add a dash of Tabasco for a bit of a kick. You want a loose, dressing-like consistency so you’re able to drizzle it over your salad. Taste and check it's to your liking. You can add more lemon and Tabasco if it needs a bit more flavour.
After 8 to 10 minutes in the oven, the salmon should be just cooked, but still slightly pink and blushing in the middle. Take it out of the oven and put to one side to cool slightly. Meanwhile, get a large serving plate or platter. Roughly shred the little gems and mix together with the watercress, dress with a little extra virgin olive oil and scatter over the platter.
Break the blushing salmon into nice big chunks and place them in and around the salad. Scatter over the cooked prawns, then spoon the Marie Rose dressing over everything. You can finish off the dish off by snipping over the salad cress. If you want to go for the old-school look, sprinkle a little paprika over the top. Serve with the remaining lemon (cut into wedges) and a glass of lovely cold white wine. Perfect!