Heat your oven to 200°C/Fan 180°C/Gas 6. Pour 1 tbsp olive oil into a large roasting tin and pop into the oven to heat up.
Peel the potatoes and cut into large, evenly sized chunks. Take the tin out of the oven and tip in the potatoes, stirring to coat with the oil. Season well, then roast for 15 mins.
Top and tail the carrots, then peel and slice into long, thick batons. Peel the beetroot and cut into wedges.
Remove the potatoes from the oven and turn over. Add the carrot and beetroot to the tin. Return to the oven for a further 30 mins.
Grate the zest of the lemon into a bowl and squeeze in half of the juice. Spoon in the mustard and crème fraîche and stir well to combine. Peel and finely grate in half of the horseradish. Have a taste, and add more for a stronger kick.
Rub 1 tsp olive oil onto all sides of each salmon and season well. Place, skin side down, on a baking tray. Roast in the oven, below the veg, for 10 mins.
Pick any thick steams out of the watercress so you just have leaves and tender shoots.
Serve the salmon with the roast vegetables, a handful of watercress and a good dollop of the horseradish crème fraiche