Dissolve the yeast in 30ml warm water in a small bowl.
In a bowl, mix the flours and salt. Mix in the yeast mixture, beer and vinegar together well until a ball forms. Cover with plastic wrap or place in a tightly closing container and leave to rise for 8 hours at room temperature.
Line a dish that is not too deep with baking paper. After allowing the dough to rise knead it a dozen times into a firm ball. Lay it into the paper lined bowl and sprinkle with vegetable oil. Cover with plastic wrap and leave to rise for 2 hours.
After 1 and a half hours place a small cast iron tin with a lid onto the bottom shelf of the oven and heat the oven to 240 degrees C.
Remove the hot tin from the oven and place the dough while still in the paper into the tin. Cut off the excess paper.
Dust the top of the dough with flour and cut across it twice diagonally with a knife. Place the lid on tip and place it in the oven. Reduce the temperature to 220 degrees C and bake for 30 minutes.
Carefully remove the lid after 30 minutes and bake for a further 10 minutes. Remove the bread from the tin straight away and leave to cool on a cake rack.