Preheat the oven to 200ºC/400ºF/gas 6. Cook the potatoes in a large pan of boiling salted water for 20 minutes, then drain and steam dry. Tip into a large roasting tray and drizzle with 2 tablespoons of oil, then add a good pinch of sea salt and black pepper. Bash and add the unpeeled garlic cloves to the tray, then roast for 20 minutes. Meanwhile, finely grate the lemon zest and pick and finely chop the rosemary. When the time’s up, scatter the lemon zest, rosemary and flour over the potatoes from a height. Toss together, then squash flat with a potato masher and roast for a final 20 minutes, or until golden and crisp.
Meanwhile, scratch the outside of the cucumber with a fork to create grooves, then finely slice into rounds. Halve and quarter the radishes, peel and very finely slice the onion, then put all this into a bowl with the mustard, vinegar and half the lemon juice. Season to taste with salt and pepper, pick and add the herb leaves, toss together, then place on top of the hot crispy potatoes, mixing just before serving. Finish with a crumbling of feta, and extra lemon juice, if you like.
I have had some fun making this salad with hasselback potatoes for added texture and crunch.