Add the potatoes to a pan of salted boiling water and cook for about 15 minutes, or until tender. Put a large frying pan on a medium heat. Season the trout on both sides with a pinch of salt and pepper. Add a good lug of olive oil to the pan and scatter in the thyme tips, followed by the trout, skin-side down (you may have to do this in 2 batches). Press down on the fish with a fish slice to help the skin crisp up. Cook for 4 minutes, jiggling the pan every now and then, and turning for the last 20 seconds or so to finish it off – you want to cook it about 90% of the way through on the skin side.
Meanwhile, in a small bowl mix the yoghurt with the juice of ½ a lemon, the horseradish and a small pinch of salt. Have a taste to check it’s hot enough – it needs a good kick. Dress the beets with a good splash of balsamic and a small pinch of salt.
Drain the potatoes then toss them with a pinch of salt and pepper and a drizzle of oil. Squash half onto each plate. Squeeze the juice of half a lemon into the hot pan of fish, then transfer 2 fillets to each plate. Top each portion with a good dollop of horseradish yoghurt, a spoonful of dressed beets and a little watercress, then drizzle lightly with extra virgin olive oil and serve.