Preheat the oven to 200°C (gas mark 6).

Finely slice the onions and cook them slowly in a heavy pan with a little vegetable oil over low heat, stirring occasionally, until soft and caramelized (at least 30 minutes).

To make the pastry, sift the plain flour into a large bowl.

Rub the cubed butter into the flour until the mixture resembles breadcrumbs.

Add the cold water gradually, using a knife to mix until a dough forms.

Do not overwork the dough.

Mix the cooked onions with balsamic vinegar, then season with salt and pepper.

Spread the onion mixture evenly on a dinner plate.

Roll out the pastry into a circle slightly larger than the plate.

Carefully lift the pastry and place it over the onions, pressing down gently over the edge of the plate.

Bake for 20–30 minutes, or until the pastry is golden and firm.

Remove the tarte from the oven and let it stand for 5 minutes.

Run a knife around the edge to trim any excess pastry.

Place a serving plate over the top and carefully flip the tarte upside down.

Serve warm or at room temperature.