Process the flour and butter in a food processor until the mixture resembles fine breadcrumbs. Add water slowly to form a smooth dough.
Knead very lightly then wrap the dough in plastic wrap and refrigerate for 30 minutes.
When ready to use, roll out on a lightly floured surface until 4mm thick
In a heavy based pan add butter and cook the sliced onions gently at a low heat for 40 mins. Stir regularly until soft and caramelised. Once cooked set to the side to cool.
Butter a 30cm fluted tart case and line with short crust pastry. Trim the edges of pastry.
Prick base of pastry with a fork in several places. Line a crumpled sheet of baking paper on top and pour on baking beads. Place in the fridge for 45 mins to rest.
Bake in a 200°C oven for 15 mins. Remove from the oven, remove the baking beads and baking paper and bake for a further 10 mins.
Remove the tart and reduce the oven temperature to 180°C.
In a medium bowl whisk eggs and cream together. Add the parmesan cheese, chives and caramelised onions and mix well to combine. Season with a little salt and pepper.
Pour the mix into the pastry tin and bake in a 180°C oven for 45 minutes until the filling has set.
Serve warm or at room temperature.