Cook leeks and onions in the butter until wilted and translucent.
Add chopped potatoes, salt and water or broth. Simmer 20 - 40 minutes until potatoes are tender and remove from heat.
Puree using an immersion blender or in batches in a blender – carefully. A fine puree is not necessary for making the bread.
Cool to 75 F and add the creams. Pour into the mixing bowl.
Add flour and top with the yeast. Mix well with wooden spoon or with a paddle attachment in a mixer. Once mixed, let rest 5 minutes.
Knead by hand or using a dough hook for 8 minutes until smooth. While kneading, adjust liquids or flour to get a tacky but not sticky dough. Turn the dough out onto counter and stretch into a rectangle. Fold the dough letter-style, top downward, bottom upward, then the sides to the middle until a package is formed. Roll into a ball.
Place in an oiled bowl or container hopefully with straight sides so that you can tell when the dough has doubled in size.
After 20 minutes, do another strech and fold and return to bowl. It should take 40 to 45 minutes to double in size at 75°F
Divide into two even portions and roll into cylinders and place each in oiled 8 x 4 loaf pans .
While rising about 30 minutes to double in volume, heat oven to 375°F.
Bake for 30 minutes until 190F – 200F internally and sounds hollow when thumped.