Remove the silverbeet stems from the leaves. Blanch the leaves in boiling salted water for 2-3 mins until tender, remove from the pot and cool. Squeeze dry and roughly chop. Set aside. In the same water, add the white wine vinegar and the stems and cook for 5-7 mins. Drain and roughly chop.
In a pan, melt the butter and add the leek and cook until soft, add the silverbeet stems and cook for a further 5 mins. Add the potatoes and the vegetable stock and cook until tender - around 10-15 mins - and any liquid has been absorbed. Roughly mash half the mix. Add the preserved lemons, chopped silverbeet leaves and crumble through the feta. Adjust seasoning to taste. Cool.
Preheat the oven to 200C.
Melt the knob of butter and add some olive oil. Put down a sheet of filo on a clean bench, brush with the oil and butter mix. Add another piece of filo and repeat. Add a third piece of filo. Cut in half width-wise. Add some of the mix in each half, fold over the edges, roll up and brush with the olive oil mix and sprinkle with the sesame seeds. Repeat with the remaining mix and put onto a baking paper-lined baking tray. Bake for 30-40 mins until golden.
1 bunch silverbeet (8-10 stems)
2 tablespoons white wine vinegar
50g butter, plus a knob of butter for brushing the filo
1 large leek, halved, washed, thinly sliced
700g agria potatoes (approx 3-4 medium), cut into 1cm cubes
1 1/2 cups vegetable stock
1/2 preserved lemon, sliced
1 packet filo pastry
olive oil for brushing the filo