Preheat oven 200°C / Gas 6.
Finely slice the onions and cook slowly in a heavy pan until really soft, turning from time to time.
This will take at least 30 minutes.
Meantime make the pastry – sift the flour into a large bowl, cube the butter and rub into the flour until it resembles bread crumbs.
Using a knife stir in just enough cold water to form a dough.
Mix the cooked onions with the balsamic vinegar and season. Tip onto an ANTA dinner plate. Roll out the pastry to form a circle slightly larger than the plate.
Carefully lift it and place over the onions and press down over the edge of the plate. Bake for 20–30 minutes until the pastry is golden and firm. Remove from the oven and let it stand for 5 minutes.
Run a knife round the edge of the plate trimming off any excess pastry, place an ANTA serving plate over the top of the pastry and carefully turn the whole thing upside down.