Cool the boiling water until it reaches 90C/194F. Quickly whisk in the rye flour trying to avoid too many lumps forming. Cover the bowl and leave for 1 hour (or up to 24 hours) . This gelatinized mixture will give the crumb of the loaf some elasticity.
Whisk the rye leaven into the cold water, then whisk in the gelatinized rye mix (saving 1 tablespoon to brush over the loaf) . Stir this with the dry ingredients until you have a soft sticky paste. Tip this dough out on to a lightly oiled work surface and knead gently into a ball. Cover and let rest for 10 minutes.
Shape the dough into a baton or a ball. Place the loaf seam-side-up on a floured cloth or baking paper, pulling the cloth up around the dough to hold its shape or proof it on a basket. Leave to rise at room temperature for 5 hours or until the loaf has almost doubled in size.
Preheat the oven to 210C/410F. Carefully upturn the loaf on to a tray and brush with the gelatinized rye mix. Spray the loaf lightly with water, and bake for 50 minutes. Leave to cool on a wire rack. When cold, wrap tightly in baking parchment, and leave for a day before slicing. Spread the bread slices with desired toppings.