Preheat the oven to 180°C. Grease a 23 cm tart pan with removable bottom.
In a bowl, combine 200 grams of all-purpose flour, 100 grams of cold, cubed unsalted butter, and 50 grams of powdered sugar. Mix until the mixture resembles breadcrumbs.
Add 1 beaten egg and mix until the dough comes together. Press the dough into the prepared tart pan, ensuring even coverage. Refrigerate for 30 minutes.
Prick the base of the tart with a fork and bake for 15-20 minutes, or until lightly golden. Remove from the oven and let it cool.
In a saucepan, combine 200 grams of granulated sugar and 60 ml of water. Cook over medium heat until the sugar dissolves and turns into a golden caramel.
Carefully add 100 ml of warm heavy cream to the caramel, stirring constantly. Be cautious as the mixture may bubble up. Cook for 2-3 minutes until smooth.
Remove the caramel from heat and stir in 200 grams of coarsely chopped walnuts. Pour the walnut-caramel mixture over the cooled tart base and spread evenly.
Bake the tart for 20-25 minutes, or until the caramel is bubbling and the walnuts are golden. Allow the tart to cool completely before removing it from the pan.