To make the tart crust, combine the flour, butter, salt and egg in your food processor.
Pulse until well combined then add 2-4 tablespoons of water to bring the dough together. Add the water gradually until the dough becomes easy to work with.
Place the dough on your floured working surface and roll it between two sheets of baking powder until it has 3-4 mm thickness.
Transfer the dough in a tart pan and press it well on the bottom and sides of the pan. Trim off the edges if needed.
Place a sheet of baking paper into the pan over the dough and fill the pan with baking beans. This is called blind baking and it basically means baking the crust before filling it. If you were to fill it while still raw, the crust would absorb part of the filling and become mushy, doughy. Blind baking ensures the crust remains crisp and perfectly baked.
Bake the crust in the preheated oven at 330F for 15 minutes.
Once the crust is baked, remove from the oven and place it aside, removing the beans.
To make the filling, combine the butter, golden syrup, treacle and heavy cream in a saucepan and melt them together over medium heat.
Remove from the heat and allow the mixture to cool down slightly.
Stir in the eggs, egg yolk, lemon juice, lemon zest and breadcrumbs and mix well. Pour the filling into the crust and bake at 330F (160C) for 30-40 additional minutes until the filling looks set.
Slice the tart when chilled and serve.
The ground hazelnuts give the tart a nutty flavor which I loved, but feel free to replace them with breadcrumbs if you like.