Prepare quinoa as per packet instructions.
Heat the butter in a frying pan over medium heat. Stir the leek for 3 minutes or until soft. Transfer to a bowl. Add the quinoa, feta, flour, lemon rind and egg and mix until well combined. Season to taste.
Shape 4 tablespoons of the mixture into patties. Add oil to a heavy based saucepan or wok (enough to shallow fry). Heat over medium-high heat. Cook patties in batches, for 2 minutes on each side or until golden. Transfer to a baking tray lined with paper towel. Cover with foil to keep warm before serving.
Assemble burger and enjoy.