Martha's Cheddar, balsamic & red onion tart

To make the pastry, place the oats into a food processor and blitz until they are floury, with just a few small pieces of oat remaining. Add the flour and cubed butter, and pulse until the mixture resembles fine breadcrumbs. Add 2-3 tbsp water and stir to combine. When the mixture clumps together, knead briefly, then wrap and chill in the fridge for at least 30 minutes, or until required (the pastry will keep for up to 2 days at this stage).

Preheat the oven to 180°C, gas mark 4 and place a 23cm fluted tart tin onto a baking tray. Roll out the pastry into a circle slightly bigger than the tin and use to line it, pressing with your fingertips so it fits snugly into all the corners. Don’t worry if there are any holes, simply patch up with any leftover pastry. Roll your rolling pin over the top of the tin to help trim any excess dough. Prick the base with a fork, line with baking parchment and fill with baking beans, rice or lentils. Blind bake the tart for 15 minutes, then remove the parchment and bake for a further 10 minutes until the base feels crisp.

For the filling, cut one of the onions fairly thinly into circular slices reserving the centres, and separate each piece into rings. Arrange the rings on a large baking tray and place into the bottom of the oven for 10 minutes to soften (you can do this at the same time as the tart case is baking).

Finely chop the second onion and any onion centres left over from making the rings and gently fry in a frying pan with the olive oil for about 5-6 minutes, until soft. Once beginning to brown, add the sugar and balsamic vinegar, and continue to cook for 2-3 minutes until sticky and reduced. Allow to cool slightly. Spread the onion mixture over the bottom of the baked tart case.

Use a fork to beat the eggs in a measuring jug, then add the crème fraîche, cream, 175g of the Cheddar and a crack of black pepper. Mix until well combined, then pour over the onions in the pastry case. Sprinkle the top with the remaining cheese and arrange the cooked onion rings in a pattern over the top.

Bake the tart for 30-35 minutes, until deep golden and the filling is risen a little, like a soufflé (this will settle as it cools). Allow to cool for a few minutes before serving hot, or allow to cool to room temperature and then chill for later. If you do refrigerate the tart, I’d recommend allowing it to return to room temperature before eating.