Make shortcrust pastry for crust first. With hands, smush butter into flour until it's all combined and crumbly. It should stick together if you squish the mixture in your fist. Add the cold water and form into a ball. The mixture should hold together. If it's still crumbly, add a little more water. Wrap in siran wrap and stick in fridge to rest for about 20 minutes.
In the meantime, zest and juice the lemons. Then cut the crust off your bread and blend into small crumbs in a food processor. Open the can of golden syrup. Place all ingredients by the stove top and take out a large skillet.
Go back to your dough. Cut about 1/3 off and set aside. Roll out the rest nice and thin. Line the dough in a 8inch tart tin, or pie dish. Prick the bottom with a fork. Cut the remainder of the dough off the edges and add to the dough you set aside. Roll out remainder of dough and cut into even 1/2 inch strips.
Preheat your oven to 400°F and go back to the stove top. Heat up your golden syrup until it starts to feel thinner on medium-low heat, but do not bring to a boil. Add the breadcrumbs, lemon zest and juice and mix all together. Take off heat once combined and pour into your prepared dish with the crust in it.
Interweave in a lattice pattern on top of the treacle filling. Cut the extra edges off. Egg wash top, but be careful to not get egg wash on the filling, only on the lattice crust.
Bake in oven for 10 minutes, then decrease the temp to 350°F and bake another 25-30 minutes. Crust should be nice and golden. Let cool a bit so the filling can set. Serve warm or cold.
*Vegan Option: If you want to make this vegan, make sure the sandwich bread you're buying is vegan. Also instead of doing an egg wash, you can brush on some milk to the crust.