Cook the potato in a large saucepan of lightly salted boiling water for 15–20 minutes until tender but not flaky. Drain really well, reserving the cooking liquid.
Put 6 tablespoons of the cooking liquid into a large bowl and leave to cool until lukewarm. Sprinkle over the yeast, then stir in the sugar and set aside for 10 minutes.
Mash the potatoes with the oil, then stir in the yeast mixture and mix well with a wooden spoon. Mix in the flours, herbs and salt and pepper, then turn out on to a lightly floured surface and knead well to incorporate the last of the flour. Knead the dough until soft and pliable, then put in a lightly oiled bowl, cover with clingfilm and leave to rise in a warm place for 1 hour until well risen.
Knead the dough on a lightly floured surface, then roughly shape into a round, place on a baking sheet and lightly cover with oiled clingfilm. Leave to prove in a warm place for 30 minutes. Score a cross into the dough with a knife and bake in a preheated oven, 220°C (425°F), Gas Mark 7, for 35–40 minutes until well risen and crusty on top. Transfer to a wire rack to cool for 30 minutes.
Cut 4 slices of the bread and lightly toast. Meanwhile, heat the oil for the topping in a frying pan, add the tomatoes and cook over a high heat for 2–3 minutes until softened. Stir in the thyme and salt flakes. Serve with the toasted bread, seasoned with pepper.