by Valentine Warner
from What to Eat Now
Preparation time less than 30 mins
Cooking time over 2 hours
14 medium onions, very finely chopped
100ml/3½fl oz white wine vinegar
2 tbsp soft light brown sugar
1 tsp Dijon mustard
2 tsp sea salt flakes
1 ready-cooked savoury pastry case
50g/2oz parmesan, freshly grated
freshly ground black pepper
1. Melt the butter in a large heavy-based non-reactive pan with a lid over a very low heat. Add the onions and cook with the lid on, stirring regularly to ensure the onions do not burn, for about two hours. The onions should be very soft, sticky and brown.
2. Turn up the heat and add the vinegar, sugar, mustard and salt. Cook until the vinegar has almost completely reduced. Stir regularly for about 30 minutes to prevent the onions burning, until the onions are a deep golden-brown. (A little colour from the bottom of the pan is fine - just stir it in.)
3. Preheat the oven to 200C/400F/Gas 6.
4. Fill the ready-cooked pastry case with the onion mixture and smooth it out evenly with a spatula. Sprinkle with grated parmesan season with freshly ground black pepper.
5. Bake the tart in the oven for 20 minutes. Remove and leave to rest for ten minutes.
6. To serve, slice while warm and serve on warmed plates.