Combine half the flour with the yeast and about 175ml of the water in a large mixing bowl. Mix well to form a very thick batter, then allow it to rest for roughly 2 hours until it has risen and then deflated.
Add the remaining flour, water, the salt, treacle, butter and caraway seeds, mixing until you have a dough. Shape into a rectangle, then dust the inside of a 2lb loaf tin with a little extra rye flour and fit the dough inside. Allow it to rise for 2 hours.
Line a baking tray with nonstick baking paper, then turn the risen dough carefully on it and bake for 30 minutes until risen. Remove and allow to cool completely on a wire rack. When ready to serve, slice thinly into rectangular slices, removing any uneven crusts and arrange stacked on a plate lined with a napkin.
Garnish with the parsley and red onion slices.
Freezing and defrosting guidelines
Bake according to the recipe. Allow to cool completely. Wrap well in clingfilm and tin foil (to prevent freezer burn) and freeze for up to 3 months.
Loosen the tin foil and clingfilm but leave to defrost in packaging at room temperature for several hours before serving.
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