Boil potato until soft. Drain and reserve cooking liquid. Rice the potato through a food mill.
Scald milk and combine with 1/2 cup reserved cooking liquid. Combine liquids and potato with lard, butter, salt, and sugar in a large bowl. Set aside to cool to room temperature.
Meanwhile combine yeast and warm water, and set aside until foamy. When potato mixture has cooled, add yeast mixture. Add flour and knead with a dough hook, or an electric mixer, until smooth and glossy, about 7 minutes.
Transfer to a buttered bowl, cover with plastic wrap, and set aside in warm place to rise about 1/2 hour. Punch dough down and briefly knead. Butter a 9 x 5 x 3-inch loaf pan. Place dough in pan, cover with plastic, and let rise until doubled, about 45 minutes.
Preheat oven to 350 degrees. Bake 30 minutes, until bread sounds hollow when tapped.