Heat 1 tablespoon oil in a saucepan, add the onion and cook for 5 minutes until softened. Stir in the garlic and cook for 1 minute more. Add the stock and simmer for 2–3 minutes, add the peas and cook for 3 minutes until soft. Reserve some of the peas, then whizz the remainder with a stick blender until nearly smooth. Stir in most of the mint and season to taste.
Rub the remaining oil over the lamb and season well. Cook under a preheated hot grill for 5-7 minutes on each side until browned and just cooked through.
Slice the lamb and arrange on serving plates with the pea mash. Scatter over the reserved peas, remaining mint and feta to serve.