Work the flour, cold butter cubes, and sugar together with your fingertips until the mixture looks like coarse breadcrumbs with some larger butter pieces still visible. Those butter bits will create the flaky layers you want in your pastry.
Beat the egg yolk with the cold water, then pour it into the flour mixture. Use a fork to bring everything together until it forms a rough dough — don't overwork it. Wrap tightly in plastic and refrigerate for 30 minutes to firm up.
⏱ 30 min
Get your oven heating to 200°C (180°C fan/400°F). The high initial temperature will help set the pastry edges and prevent shrinkage during blind baking.
Roll the chilled pastry on a floured surface until it's large enough to line your 23cm tart tin with some overhang. Press it gently into the corners and trim the edges, leaving about 1cm above the rim.
Line the pastry with parchment paper and fill with ceramic baking beans or dried pulses. Blind bake for 15 minutes until the edges are just starting to color — this prevents a soggy bottom later.
⏱ 15 min
Carefully lift out the parchment and beans, then return the pastry to the oven for another 5 minutes. You want it pale golden and dry to the touch before adding the filling.
⏱ 5 min
Gently warm the golden syrup in a heavy-bottomed saucepan over low heat, stirring occasionally, until it flows easily off the spoon. Don't let it bubble or it may become too thick.
Remove the syrup from heat and stir in the breadcrumbs, lemon juice, and zest. The mixture should hold together but still pour smoothly — the breadcrumbs will absorb some syrup and provide texture.
Pour the warm filling into your prepared pastry case and use the back of a spoon to spread it evenly to the edges. The surface should be fairly level for even baking.
Lower the oven temperature to 180°C (160°C fan/350°F) and bake the tart for 15-20 minutes until the filling is set but still has a slight wobble in the center when gently shaken.
⏱ 20 min
Let the tart cool completely in the tin before attempting to slice — this allows the filling to firm up properly. Serve at room temperature with a dollop of clotted cream if you're feeling indulgent.