Heat olive oil in a large frying pan over medium-high heat, add lamb and brown well all over, breaking up any clumps with a wooden spoon (2-3 minutes). Transfer to a bowl using a slotted spoon and pour off all but 2 tbsp fat from the pan. Add onion, fennel, garlic and lemon rind, sauté until translucent (4-5 minutes), then stir in cumin, coriander, chilli and dried mint, and fry until fragrant. Return lamb to the pan, add tomatoes and stock, season to taste, bring to a simmer and cook until liquid reduces by half and lamb is well flavoured (10-12 minutes). Stir in fresh herbs and olives and check seasoning.
Meanwhile, cook orzo in a large saucepan of well-salted boiling water until just tender (4-5 minutes), then drain and toss in a bowl with extra-virgin olive oil and lemon juice.
To serve, divide orzo among serving bowls, top with lamb mixture and serve hot scattered with feta and fennel fronds.