Red Onion and Cheese Tart

500g (1 block) frozen short crust pastry
1tbsp olive oil
1tsp caster sugar
1tbsp butter
1tbsp balsamic vinegar
500g red onions thinly sliced
3 large eggs
300ml single cream or full milk
125g grated strong cheddar

Roll out the pastry and use it to line a 23cm flan/quiche tin. Price the base and bake blind (cover with baking paper and use cooking beans to weight it down) for 1o mins. Uncover and cook for further 4 mins. Remove from oven and leave to one side to cool.
Meanwhile fry the onions in a large frying pan with the oil and butter until soft. Add the sugar and balsamic vinegar and cook for further few minutes until deep brown. Place the onions into the flan and season. Beat together the eggs and cream/milk and add the grated cheese. Pour over the onions and bake in oven 2oo/180/6 for 30 mins
Make sure the flan sits flat in the oven unlike mine which slopped over a bit