Preheat the oven to 190°C.
Line the ready rolled puff pastry into a 20cm loose bottomed quiche or tart tin approximately 30cm x 12cm tin. Prick the base of the pastry, cover with crushed baking paper and add some baking beads or rice to hold the paper down. Stand for about 20 minutes then pop the tart shell into the heated oven and bake for 15 minutes. Set aside to cool.
To cook the onions, slice them finely. Heat the butter in a small pan and when it starts to foam, toss in the onions with the salt, pepper and sugar. Cook over gentle heat so they become soft and eventually turn golden. After about 30 minutes add the vinegar and allow this to become absorbed.
Set aside to cool.
Beat the mascarpone, eggs and milk together until smooth, then crumble the blue cheese into the mixture with a little extra salt and pepper.
Spread the onion mixture over the base of the prepared tart shell. Pour in the cheesy mixture and bake for about 20-25 minutes until firm and the pastry has become golden and crisp. Serve with sliced fresh tomatoes.
Wine match: sauvignon blanc
For more information: