1. Whisk the starter ingredients together in a medium bowl. Cover with plastic wrap and let stand for 8–12 hours, until bubbly.

2. Transfer to a large bowl. Stir in the rye flour and salt. Stir in enough of the bread flour to make a stiff dough. Knead on a lightly floured work surface, adding more flour as needed, about 8 minutes, until the dough is smooth and elastic, but slightly sticky'do not add too much flour. Shape the dough into a ball, and turn in an oiled bowl. Cover with plastic wrap and let stand in a warm place until the dough doubles, about 1 hour.

3. Punch down the dough and form into a 9in (23cm) long football shape. Transfer to a floured baking sheet, cover with plastic wrap, and let stand until almost doubled, about 30 minutes.

4. Preheat the oven to 425°F (220°C). Brush the loaf with beaten egg and sprinkle with caraway seeds. Cut a shallow slash down the center of the loaf. Bake for 20 minutes. Reduce the oven temperature to 400°F (200°C) and continue baking until the dough sounds hollow when tapped on the bottom. Cool on a wire rack.