In a large bowl place the flour, oats, baking powder, salt and chopped coriander leaves. Stir together then add the cubed butter. Rub the butter into the dry ingredients until mealy with a few larger chunks of butter remaining. Chill this while you prep the wet ingredients.
To the jug of candied lemon zest & syrup, add the yogurt and stir together. Crack in the egg and stir that in well, too. Pour this into the chilled dry ingredients and stir together just until no floury patches remain.
Place the dough into the freezer for 10 minutes while you preheat the oven to 350 F (180 C) and line a baking tray with parchment paper and sprinkle lightly with flour.
Transfer the dough to the lined, floured tray and sprinkle with a bit more flour. Pat the mixture out into a rectangle 2 cm thick. Cut into 6 squares and spread them around the baking tray. Brush with milk and sprinkle with granulated sugar (if you want) then bake for 25 minutes until golden.