Preheat oven to 400 degrees F/ 200 degrees C. Adjust the oven rack at a middle position.

Place the butter in a heatproof bowl and melt it in the oven for few minutes.

Combine the two kinds of flour in a bowl and add salt. Mix to blend.

In another bowl combine yogurt, water, sugar and baking soda. When the mixture starts bubbling, add also the melted and cooled butter.

Pour the mixture into the bowl with the flour and start mixing with a wooden spoon. When the flour is absorbed, knead with your hand for up to 1 minute.

The dough will be sticky so sprinkle it with some flour. Make a round loaf and place it on a sheet of baking paper. Extend it a bit using a rolling pin or your hands. Using the handle of the wooden spoon or a sharp knife make a cross through the middle of the loaf.

Sprinkle with coarse sea salt and bake it in a large tin.

Test for doneness in 25 minutes: it is ready when a toothpick inserted in the centre comes out clean, the bottom crust looks golden brown and when knocking on the bread it makes a hollow sound.

Place the bread on a wire rack and cover it with a clean towel.