Cook the potatoes, mash and cool them. Cook the courgettes with a little of the olive oil on low heat until softened and translucent, but not mushy.
In the meantime, mix the yeast with 50ml of the warm water and 2 tablespoons of the white flour, leave covered for 30 minutes.
Add the onions and the mash to the yeasty mixture; add the rest of the water and white flour, the rye flour, salt, sugar and caraway seeds and mix to combine.
Leave the dough standing for 10 minutes, then knead by hand or in a standing mixer with a dough hook attachment adding the rest of the oil. If the dough looks very sticky and doesn’t want to come together, add the wholemeal flour. Continue kneading until it’s smooth, stretchy and bounces off the sides of the bowl or stops sticking to your hands. Add the fresh chopped herbs and mix them in thoroughly.
Place the dough in a large bowl (it will rise quite a lot), cover with cling film and leave in the fridge overnight.
The next day flatten the dough to a rectangle and roll it up tightly, to the width of a loaf tin. Drop it into a buttered and floured tin, cover with oiled cling film and leave in a warm place to expand in volume by half – 1-2 hours.
Preheat the oven to 220C/425F/gas 7. Dust the top of the loaf with flour, slash with a sharp knife down the middle and bake for 35 minutes. Then lower the heat to 180C/350F/gas 4 and bake for further 10 minutes. Cool on a wire rack.
