Pre- heat oven to 200C/180C fan/gas 6.
In a bowl, toss the beetroot, onion and clementines in 2 tbsp of the oil, the vinegar and sugar.
Season with salt and pepper and mix.
Heat the rest of the oil in a large, ovenproof non-stick frying pan. (or use any round ovenproof dish here)
Add the veg/fruit mixture to the frying pan so that the surface of the pan is covered.
Cover the frying pan with foil and cook in the oven for 45 mins.
Roll out the ready made pastry on a floured surface. A thickness of 1/2cm should do and about the same size as your frying pan.
Now carefully take the pan out of the oven and remove the foil.
Put the pastry on top, tucking it in all around the edges.
Return the pan to the oven and bake for 35 mins or until the pastry has puffed up and is a deep golden brown.
Remove the frying pan from the oven.
Slide a palette knife around the edge of the tart,(or a fish slice) then put a plate on top of the pastry, serving side down.
Flip the pan over to turn the tart out onto the plate.
Top with clementine zest and some greenery to be healthy.