Grate the potato or mash roughly and combine in a large bowl with the flour, salt and yeast. Make a well in the centre and add the soured cream and water. This dough takes slightly longer to get going and rise than others because the dough temperature is lowered by the cold soured cream. If you can’t wait, the water can be a little warmer than lukewarm – aim for hand hot. Mix together to form a rough dough.
Knead for 10 minutes until the dough turns smooth and elastic. There will still be lumps of potato in there so it isn’t supposed to get completely smooth, don’t worry! Transfer to a lightly oiled bowl, cover and allow to rise for 1½ hours or until doubled in size.
While the dough rises add the smoked bacon lardons to a frying pan and cook until lightly golden. Transfer to a bowl and allow to cool.
Turn the risen dough out onto the work surface and pat it with your hands to knock it back and spread it out a little. Scatter over the cooked bacon lardons and the grated cheese. Gather the dough together and knead gently for a couple of minutes to incorporate the cheese and bacon evenly. Place it back in the bowl to rest for 5 minutes – this will make the shaping easier.
Line a colander with a cotton dish towel and dust liberally with flour.
Take the dough out of the bowl and pat it out once again. Fold each outside edge in to the centre, repeating all the way round. Repeat again, folding the outside to the middle until the dough is tightened up into a rough ball. Pop the ball of dough into the lined colander with the seam upwards.
Fold the overhanging edges of the dish towel over the exposed dough and place in a warm position to rise until almost doubled in size, approximately 45 minutes. Preheat the oven to 190°C (fan)/210°C/425°F/Gas Mark 7.
Line a baking sheet with baking (parchment) paper or grease well. Unfold the dish towel and place the tray over the top of the colander. Invert the tray and colander together then remove the colander and dish towel. Dust the loaf with a little more flour and slash the top in a square pattern using a very sharp knife.
Bake in the preheated oven for 35 minutes. When it’s baked through remove from the oven and transfer to a wire rack to cool completely. The crust will be firm as soon as it comes out of the oven but will soften on cooling to leave you with a tender loaf and crust.