Place the sourdough, melted butter, strong white bread flour and sea salt into a bowl. Mix until a firm dough forms.
Add the rosemary or spices, if using, and mix until evenly combined.
Tip onto a lightly floured work surface and roll out as thinly as possible.
Transfer to a parchment-lined tray. Sprinkle with a little extra sea salt and any additional herbs or spices, if using.
Cut or score into shards.
Bake in a preheated oven at 180°C for 15 minutes, until golden, crisp and evenly coloured.
Leave to cool completely before serving.