In a large bowl, combine the flour, salt, and butter. Rub the mixture with your fingers until it resembles fine breadcrumbs. Add the egg and cold water, kneading just until the dough comes together. Shape the dough into a ball, wrap it in cling film, and refrigerate for 30 minutes.

Slice the onions into thin half-rings. Melt the butter in a pan over medium heat and sauté the onions until soft and caramelized (about 20 minutes). Stir frequently to prevent burning. In a bowl, whisk together the eggs and cream, then add the grated cheese, salt, pepper, and nutmeg. Stir to combine.

Roll out the chilled dough and fit it into a tart pan (about 23 cm in diameter). Press the edges and prick the base with a fork. Pre-bake the crust at 180°C (350°F) for 10 minutes using blind baking (cover with parchment paper and add pie weights or dried beans). Spread the caramelized onions evenly over the crust, then pour the cheese and cream mixture on top. Bake for 25-30 minutes until the filling is golden and set.

Let the tart cool slightly before slicing. It’s delicious both warm and cold—perfect for serving at a party when you don’t want to be stuck in the kitchen!