Preheat oven to 220°C (200°C fan-forced, 375°F). Line a loose-base tart tin with baking paper.
Remove silver beet stalks and discard. Chop leaves roughly. Finely chop the spring onion.
Place pastry in the prepared tin, pressing it into edges. Trim off excess pastry and prick the base with a fork. Line pastry shell with baking paper and pour in some baking weights or uncooked rice. Bake for 5 minutes. Remove from oven, carefully remove paper and weights, then return pastry shell to the oven for 5 minutes. Remove from oven and place on a wire rack.
Meanwhile, steam silverbeet in a steamer or a metal colander set over a saucepan of boiling water for 5 minutes. Turn off the heat. Drain water out of the pan. Using a wooden spoon, squeeze excess water from silverbeet and return it to the pan. Add remaining ingredients and mix well with a spoon.
Spoon mixture into the pastry shell and smooth the top. Bake on the middle shelf in the oven for 15 minutes. Remove from oven, place on a wire rack to cool for 5 minutes, then transfer to a serving plate. Serve warm. Leftover tart can be stored in an airtight container in the fridge for up to 5 days or wrapped snugly in baking paper and frozen for up to 6 months. The leftover tart can also be enjoyed cold. Leftover tart can also be enjoyed cold.
Leftover tart can be stored in an airtight container in fridge for up to 5 days or wrapped snugly in baking paper and frozen for up to 6 months.