Scrub the potatoes. Quarter or halve them if they’re large so they’re all roughly the same size. Tip the potatoes into a pan. Cover with boiling water. Add a pinch of salt. Cover, bring to the boil, then simmer for 15 mins till the potatoes are tender when you press them with a fork.
While the potatoes cook, peel and crush the garlic. Scrape it into a small food processor. Finely grate in the zest from the lemon and squeeze in the juice.
Add the tahini, 1 tbsp olive oil and a good pinch of salt and pepper. Whisk everything together. Taste and add more salt or pepper if you think it needs it. Set aside.
Trim the courgette. Peel ribbons off the courgette with a vegetable peeler till you get down to the seeded core, then finely slice that. Pop them in a bowl. Trim the roots off the spring onions and finely chop them. Add to the courgettes with the lamb's lettuce.
Drain the potatoes. Cool them under cold running water so they’re warm rather than hot. Add them to the veg and pour in the green tahini dressing. Toss to mix the salad together. Put to one side.
Heat your grill to high. Line the grill pan with foil. Brush the trout fillets with ½ tsp olive oil on each side. Season with salt and pepper. Pop on a rack in the grill pan.
Grill the trout, skin-side up, for 8-10 mins till the skin is crisp and lightly golden. Serve with the salad.