To begin, dissolve the sugar into the potato water and sprinkle yeast on top. Allow it to proof for about 5 minutes, until active and frothy. Mix in the "sour cream" or "yogurt," and begin to work in the flour, 1 cup at a time. Use the dough hook on your stand mixer if you have one, and allow it to kneed slowly and create a sticky but workable dough. Err on the side of using less flour for now; You can always work more into it later.
Let the machine continue to knead the dough for about 10 minutes on low speed. Cover the bowl, stash it in the fridge, and allow it to sit for 12-24 hours. Yes, that long! Your patience will be rewarded.
Lightly grease a 9 x 5-inch loaf pan, and set aside. Turn out the dough onto a lightly floured surface, and have more flour ready if needed. Knead the dough by hand, adding more flour if too sticky, for about 10 minutes. Let it rest for another 10 so that the gluten can relax a bit before shaping. Flatten the dough into a rectangle slightly shorter than 9 inches, and then roll it up tightly. Place the roll seam-side down in your prepared loaf pan.
Allow the loaf to sit for 60-90 minutes, until just barely peeking out above the rim of the pan. Bake in a 375 degree preheated oven for 35-40 minutes, until golden brown. Let cool on a wire rack completely before slicing.