Step 1Place the butter and flour in the bowl of a food processor and process until it resembles fine breadcrumbs. Add egg yolk and water, and process until dough just starts to come together. Turn dough onto a lightly floured surface and knead until smooth. Shape into a disc and place in the fridge for 30 minutes to rest.
Step 2Roll pastry into a 15 x 40cm rectangle, 3mm thick. Line a shallow 10 x 34cm (base measurement) fluted tart tin, with removable base, with pastry. Trim excess. Place in the fridge for 30 minutes to rest.
Step 3Meanwhile, preheat oven to 200°C. Melt one-third of the extra butter in a non-stick frying pan over medium-high heat until foaming. Add half the pear and cook, turning occasionally, for 4 minutes or until golden. Sprinkle with half the sugar and cook, turning occasionally, for 2 minutes or until caramelised. Transfer to a plate. Repeat with half the remaining butter, and the remaining pear and sugar.
Step 4Melt the remaining butter in the same pan over medium heat until foaming. Add onion wedges and cook for 2-3 minutes or until golden brown. Transfer to a plate.
Step 5Cover pastry base with baking paper and fill with rice or dried beans. Place on a baking tray. Bake in oven for 10 minutes. Remove paper and rice or beans, and bake for a further 10 minutes. Remove from oven. Reduce temperature to 160°C.
Step 6Whisk together the cream, egg and sage in a jug. Season with salt and pepper. Arrange the pear, onion and brie over pastry base. Pour in cream mixture. Bake in oven for 30 minutes or until just set. Remove from oven and set aside for 5 minutes. Cut into slices to serve.
