Place the feta and 125 ml (4 fl oz/½ cup) water in an upright blender and blend until smooth. Add the methylcellulose. Soak the gelatine in cold water for 5 minutes or until softened. Squeeze out the excess water, then melt in a double boiler over simmering water. Add to the feta mixture and combine. Spread over a large tray lined with freezer film to 1 mm (1/32 inch thick). Place in the freezer and chill until 1–4°C (34–39°F), then cut into twelve 4 cm (1½ inch) squares. Set aside. Bring a saucepan of water to the boil. Blanch the silverbeet for 1 minute. Drain, refresh in iced water, drain again and squeeze out the excess water. Chop finely and set aside. Place the quinoa in a small saucepan and cover with water. Cook over medium heat for 20 minutes or until softened. Strain and add to the silverbeet with the garlic, mint and preserved lemon and stir to combine.
Place a piece of feta on a 30 cm (12 inch) square piece of plastic wrap. Top the feta with 15 g (½ oz) of the silverbeet mixture. Take all 4 corners of the plastic wrap and twist together to form a tight ball and tie off to secure. Repeat to make 12 balls. These can be made 1 day in advance and refrigerated. When ready to serve, heat a saucepan of water to 80°C (176°F) and poach the balls for 4 minutes. Scoop out and remove the plastic. Set aside.