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Heat 1 tbsp oil in a pan or flameproof casserole dish over a medium heat. Add the onion and cook for 3 mins, then add the garlic and lemon zest and fry for another 1 min. Add the pearl barley and the stock. Season, bring to the boil, then simmer for 30 mins, stirring occasionally.
Meanwhile, put the feta in a bowl with the remaining olive oil, half the lemon juice, most of the mint and a good grinding of black pepper. Leave to marinate while the barley cooks.
Remove the lid from the barley and cook for 5 mins more. Increase the heat then add the peas, half the feta and all the feta juices. Cook for 3 mins, then check the seasoning. Divide between four bowls and top with the remaining feta and mint.