1. Place the flour in a large mixing bowl and make a well in the center.
2. Pour ¼ cup of the warm water into the well. Add the yeast and sugar and stir gently to dissolve. Let sit for 5 to 10 minutes until it is bubbling.
3. Add the oil, mashed potato, and salt. With a wooden spoon (or better yet, your hands), start to mix the flour into the yeast mixture in the well. Gradually add more of the remaining 2 cups warm water as needed to moisten the flour (being careful not to add too much—the dough should remain firm and you probably will not need to use all 2 cups!), while continuing to mix.
4. Remove the dough from the bowl and put on a floured breadboard (or a clean countertop). Knead by hand (press the dough hard with the palm of your hand, fold the dough over, and repeat) until all the flour is incorporated and the dough is well blended and smooth.
5. Wash and dry the mixing bowl and grease lightly with oil. Return the dough to the bowl, cover with a slightly damp kitchen towel, and place in a warm spot (such as in an oven that has been warmed on low, then turned off ). Let it rise until doubled in size, 1 to 2 hours.
6. Punch down the dough in the bowl. Return to the floured breadboard (or countertop) and knead until smooth.
7. Lightly oil a baking sheet. To make 1 extra-large loaf, cut the dough into 3 equal parts and roll each part into a rope of equal length. Line up the 3 ropes in a parallel row. Pinch the ends together at one end. Cross the left-hand rope over the middle rope (the left-hand rope now moves to the middle position). Cross the right-hand rope over the middle rope. Continue crossing left- and right-hand ropes until you reach the end of the ropes. Pinch the ends together and tuck under. Place on a baking sheet. To make 2 medium loaves, cut the dough in half and follow instructions for 1 extra large loaf.
8. Cover the loaves with the damp kitchen towel. Return the baking sheet to the warm spot and let the dough rise again until doubled in size, 1 to 1½ hours.
9. Preheat the oven to 350°f. Brush the top of the bread with the beaten egg white and sprinkle generously with the poppy seeds. Bake for 30 to 40 minutes, or until the top is golden brown and when you tap the bottom it makes a hollow sound. Place on a wire rack to cool.
Excerpted from THE GERMAN-JEWISH COOKBOOK: Recipes and History of a Cuisine by Gabrielle Rossmer Gropman and Sonya Gropman, published by Brandeis University Press.