Mix soaker ingredients together and let rest in covered bowl for at least 6 hours.
To bake bread, soften butter, finely mince onions and dissolve yeast in a separate bowl with a pinch of sugar and 4 Tablespoons Water. Set aside 90 g Rye flour. Combine and mix together all bread ingredients, long with soaker. Dough will be very sticky – this is normal. Add rye flour, mixing thoroughly.
Leave dough, lightly oiled, in a covered bowl. Leave dough to rise.
Once doubled in size, shape into a loaf and slash top. Lightly dust with flour. Let rest for 15 minutes. While dough is resting, preheat oven to 220 C (425 F).
Cook bread for 30-45 minutes until bread is lightly toasted and sounds hollow when tapped on the bottom.