Other varieties include a white onion and asparagus version in season; a chorizo and chipotle version in the autumn, with chiles and refried beans for a Mexican kick; and a taleggio, mushroom, and thyme version.
250g puff pastry (packet is fine)
1 beaten egg
4 medium red onions, thinly sliced
1tbsp balsamic vinegar
1tsp granulated sugar
salt and pepper to taste
200g charcuterie (optional, to serve)
1. Preheat oven to 220°C. Roll out puff pastry into a rectangle approximately 4mm thick and put on a floured baking tray. Using a sharp knife, score a border in the pastry, roughly the thickness of your thumb away from the edge. Inside the border, use a fork to pierce dozens of tiny holes.
2. Using a large frying pan over a low heat, heat around 1tbsp of olive oil and begin softening the onions. This will take around half an hour, and you will need to stir them often.
3. While the onions are softening, pre-bake the pastry base. Cook it for approximately 4-5 minutes, then remove it from the oven and press down the centre - watching for escaping steam! - as needed. Turn the baking tray 180° and return to the oven for another 4-5 minutes. Again, press down the centre as needed.
4. About 20 minutes into the softening of the onions, sprinkle the teaspoon of sugar and the tablespoon of balsamic vinegar into the frying pan. Stir well. After a final 10-15 minutes, the onions should be soft and caramelised. Season with salt and pepper and set aside to cool for about five minutes.
5. Once cooled slightly, begin spreading the caramelised onions inside the scored border on the puff pastry sheet. Thinly slice the reblochon and place it on top of the onions. Paint the pastry border with the beaten egg, and bake for 20 minutes.
6. While the tart is cooking, arrange the charcuterie (if using) on a plate or board to serve with the tart.
Serves 4 to 6 as a light lunch or supper.