Cut the sheet of pastry into 4 x 12cm diameter circles. Fit each circle of pastry into an individual fluted tart shell 10cm in diameter. Place into the freezer for 10 minutes. Leave a bit of pasty hanging over edge in case pastry shrinks.
Line each tart with alfoil, fill with raw rice or beans and bake for 8-10 minutes until pastry is set and lightly coloured. Remove rice/beans and alfoil and allow to cool.
Peel and thinly slice the onions. Heat the oil in a large pan and cook the onion on a low heat until caramelised.
Crumble 200g of the goats cheese into a bowl and mix with the mascarpone, eggs and pepper.
Place a layer of caramelised onion in the bottom of each tart shell then fill with the goats cheese mix. Bake for 15-20 minutes until tarts are set and golden in colour.
Serve warm with the rest of the goats cheese crumbled on top and garnish with the chives.