Scrub the potatoes. Quarter or halve them if they’re large so they’re all roughly the same size. Fill a pan with boiling water. Add the potatoes. Simmer for 15 mins till the potatoes are tender when you press them with a fork.
Trim the leaves off the radishes. Shred them. Quarter or halve the radishes. Pop them all in a bowl.
Slice any thick stalks off the rainbow chard. Finely shred the leaves. Add them to the radishes.
Finely slice the mint leaves. Add them to the veg. Halve the lemon. Juice one half. Whisk the lemon juice with 1 tbsp olive oil and some salt and pepper to make a dressing. Taste and adjust the seasoning.
Drain the potatoes. Cool them under cold running water so they’re warm rather than hot. Add them to the veg with the lemon 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 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 and the remaining lemon sliced into wedges for squeezing.