Caramelised Onion Tart

Nutrition · per serving

Steps

1. 1

   Preheat the oven to 200C. Lightly flour the work surface, roll the all-butter shortcrust pastry to about 3 mm thick, and line a 20 cm tart tin. Press it neatly into the corners without stretching, then trim the excess. Chill for 10 minutes so the pastry holds its shape in the oven.

   180 g all-butter shortcrust pastry10 g plain flour, for dusting
2. 2

   While the pastry chills, melt the butter with the olive oil in a wide frying pan over medium heat. Add the sliced onions, sugar, thyme leaves, salt, and pepper. Cook for 18-20 minutes, stirring often, until the onions collapse, turn deep golden brown, and taste sweet rather than sharp. If they start catching, lower the heat and stir more frequently; proper caramelisation should be rich and jammy, not burnt.

   400 g brown onions, thinly sliced25 g unsalted butter10 ml olive oil8 g caster sugar2 tsp fresh thyme leaves1/2 tsp fine sea salt1/4 tsp black pepper
3. 3

   Prick the chilled pastry base lightly with a fork. Bake for 12 minutes until the edges are just starting to colour and the base looks dry. This quick blind-free bake works because the tart is small, but the shell should look set before filling.

4. 4

   Whisk together the egg, double cream, and Dijon mustard until smooth. Stir in half of the grated Gruyere to help the custard bind and add savoury depth.

   1 large egg80 ml double cream1 tsp Dijon mustard80 g Gruyere, coarsely grated
5. 5

   Spread the caramelised onions evenly over the hot pastry shell. Pour over the custard, then scatter the remaining Gruyere and the extra thyme leaves on top. Keep the filling level just below the rim to avoid spills.

   80 g Gruyere, coarsely grated1 tsp fresh thyme leaves
6. 6

   Return the tart to the oven and bake for 16-18 minutes until the custard is just set, the cheese is melted, and the top is golden in patches. The centre should have only a slight wobble when gently shaken.

7. 7

   Rest the tart for 5 minutes before removing it from the tin and slicing. Serve warm, when the filling is set but still soft and creamy.

Origin & History

Onion tarts have long appeared in British baking, especially as savoury luncheon dishes and tea-table fare. This version reflects modern British cooking, combining French-influenced Gruyere and custard with the classic sweetness of slowly cooked onions in a shortcrust case.