Caramelised Onion and Gruyere Tart

Ingredients

• 320 g shortcrust pastry
• 6 onions, sliced thin
• 40 g butter
• a few sprigs of thyme
• 1 tbsp balsamic vinegar
• 3 eggs
• 200 ml double cream
• 100 g gruyere, grated

Method

1. Blind bake the case

   Line a 23 cm tin, chill it for 20 minutes, then bake lined with paper and beans at 190°C for 20 minutes, and 10 more with the paper off. A raw case under a wet filling never crisps up later.

   30 min

2. Caramelise the onions

   Cook the onions in the butter with the thyme and a pinch of salt over a low heat, stirring occasionally, until they collapse to a deep brown jam. This genuinely takes 40 minutes and there is no way to hurry it — high heat gives you burnt edges and raw middles.

   40 min

3. Deglaze

   Stir in the balsamic and let it disappear, scraping the sticky base of the pan into the onions. That is concentrated onion sugar and it belongs in the tart.

4. Make the custard

   Beat the eggs with the cream and most of the cheese and season well. Cream rather than milk — a milk custard weeps water as it cools and slackens the pastry.

5. Fill and bake

   Spread the onions in the case, pour the custard over, scatter the last of the cheese and bake at 170°C until just set with a faint wobble at the centre.

   30 min

6. Rest

   Rest at least 20 minutes before cutting. A hot custard tart collapses on the knife; a warm one holds its edge.

   20 min