Sift the flour, salt, sugar, and baking soda into a bowl. Make a well in the middle, add the yeast, and slowly pour in the warm milk and water mixture. Draw the flour into the center, mixing thoroughly, and beat to a thick batter. (You may need to add up to another ½ cup warm water if the mixture is very thick.) Whisk for a couple of minutes until smooth. Cover with a clean dish towel and leave to rise in a warm place for about 1 hour, until it has almost doubled in size and has a spongy consistency.