In a mixing bowl, combine the sourdough starter, white flour, oil, salt and sugar and knead into a dough. Divide the dough into 2 portions, each for one tray of crackers.
On a lightly floured surface or directly on a piece of baking parchment, roll out the dough ⅛ inch (3mm) thick, using a rolling pin.
Sprinkle on any optional toppings and roll to press them into the dough with the rolling pin.
Using a pizza cutter or a knife, cut the sheet of dough into squares (or any shape you would like your crackers to be).
Prick each cracker with a fork a couple of times, to keep them from puffing up like a pillow.
Transfer to a lined baking sheet and bake at 200°C (390°F) for 12-15 minutes, until the crackers are light golden brown.
Sourdough starterThe recipe is based on a sourdough starter fed equal parts (in weight) water and plain white (all purpose) flour. Also known as 100% hydration.If your starter has been fed in a different water to flour ratio, wholewheat flour or another different type of flour, you will have to adjust the amount of flour to get the right consistency for the dough.ToppingsYou can vary toppings to your taste, or mix and match. Here are some of our favourite toppings and quantities, for the whole recipe (two baking sheets) respectively:Sesame & nigella seeds: 1½ tsp sesame, ½ tsp nigella (2 tsp total)Sea salt & black pepper:⅛ - ¼ tsp salt, ¼ tsp pepperRosemary: 1 tsp dried rosemaryCaraway: 2 tsp whole caraway seeds