To make the pesto, put the mint, zest, garlic and oil in a food processor and pulse for a minute. Add a squeeze of lemon juice and pulse to a smooth paste. Add the remaining juice and season.
Rub the oil over the lamb and season. Cook on a smoking hot griddle pan for 5-7 minutes each side, until cooked through. Mix the lentils with the radishes, tomatoes, spring onions, 1 tablespoon of pesto and the salad leaves, then divide between 4 plates. Cut the lamb into slices and serve on the lentils, topped with some crumbled feta and pesto.
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