Potato, onion and gruyere tart

Julia Busuttil Nishimura’s potato, onion and gruyere tart.William MeppemI always have a few packets of good-quality, all-butter pastry in the freezer. I love making my own flaky pastry, but it’s reassuring to know I can create something beautiful with minimal effort when time is limited. This tart is rich and comforting and is the perfect lunch served with a salad, or it can be cut into smaller pieces and served as a canape or snack. Sometimes I use comte instead of gruyere, which is also delicious.Ingredients2 tbsp extra-virgin olive oil, plus extra for drizzling3 onions, finely slicedsea salt2 garlic cloves, roughly chopped3 thyme sprigs, leaves picked, plus extra leaves to serveblack pepper300g nicola potatoes, peeled and finely sliced125g mascarponefreshly ground nutmegfinely grated zest of 1 lemon80g gruyere or comte, coarsely grated1 × 375g butter puff pastry sheet, thawed1 egg, for egg washMethodStep 1Preheat the oven to 190C fan-forced (210C conventional) and line a baking tray with baking paper.Step 2Heat the olive oil in a frying pan over low–medium heat, add the onion and a pinch of salt and cook, stirring occasionally, for 10–12 minutes or until the onion is very soft and lightly golden. Add the garlic and thyme and cook for another 3–4 minutes. Season well with salt and pepper and set aside to cool.Step 3Combine the potato with the mascarpone, nutmeg, lemon zest and gruyere in a large bowl. Season well and toss to coat.Step 4Lay the pastry on the tray and use a sharp knife to score a 4–5cm border around the edges.Step 5Spread the cooled onion on the pastry within the border, then top with the potato mixture. Drizzle with a little extra olive oil.Step 6Whisk the egg with 1 teaspoon of water in a small bowl and brush over the border of the pastry. Bake for 30–35 minutes or until the pastry is golden and the filling is bubbling. Allow the tart to cool and top with the extra thyme leaves before serving.