In a large bowl, dissolve the yeast in the warm potato water.
Stir in the warm mashed potatoes, molasses, and 2 cups (480 ml) of the whole wheat flour.
Blend until a smooth, thick batter (called a sponge) forms.
Cover the bowl with a light towel and set it in a warm, draft-free spot for 1½ hours to rise and become bubbly.
Gently deflate the risen sponge.
In a separate bowl, whisk together the dark rye flour and salt.
Add this to the sponge along with the wheat germ, chopped prunes, golden raisins, currants, and walnuts.
Stir until everything is well combined.
Turn the sticky dough out onto a generously floured surface. Begin to knead, gradually adding as much of the remaining 2 cups (480 ml) of whole wheat flour as needed to create a firm, manageable dough.
Continue kneading for 8-10 minutes, until the dough feels elastic and springs back when you press it.
Shape the dough into a ball and place it in a large, lightly buttered bowl, turning it once to coat the top.
Cover with a towel and let it rise in a warm place for 1½ hours, or until it has doubled in size. Punch the dough down, reshape it into a ball, and let it rise again for another 45 minutes.
Divide the dough into two equal portions and shape each into a round, rustic loaf (a boule).
Place the loaves, smooth side up, on buttered baking sheets.
Cover them loosely with a towel and let them rise for a final 45 minutes.
Preheat your oven to 375°F / 190°C.
In a small bowl, whisk together the egg yolk and water to create the glaze. Gently brush this glaze over the tops of the loaves.
Bake for 1 hour, or until the loaves are deep brown and sound hollow when tapped on the bottom.
Transfer the loaves to a wire rack and allow them to cool completely before slicing.
Patience is key for the best texture!