Place the ferment in a bowl and mix in enough flour to make a dough stiff enough to roll into a log. Cut into 8 pieces.
Coat the pieces generously in flour and roll into thin circles with a rolling pin. Sprinkle the circles with a little sea salt and gently roll the salt into the crackers with the rolling pin. Use a 2-3cm cutter to remove the centre of the crackers.
Your wood-fired oven needs to be at around 210°C with a little flame presence before you can carefully transfer the crackers to a floured peel and slide onto the deck of the oven.
Bake the crackers for around 2 minutes or until just starting to colour on the edges.