Make Filling

1. Place cut potatoes in 2 qts of cold water in medium saucepan. Bring to boil, and cook until fork-tender, about 10-12 minutes
2. Meanwhile, melt chicken fat over medium heat in medium sauté pan. Add onions and sauté until softened and slightly browned, about 10 minutes. (If using hot water and bouillon, add bouillon to water to dissolve)
3. Drain water from potatoes and mix together potatoes, onions, salt & pepper, butter and milk, mashing and mixing to a smooth consistency. Spread mixture on baking sheet and cool to room temperature

Assemble Knishes & Bake

1. Preheat oven to 375º
2. Lightly dust workspace with flour. Lay one sheet of puff pastry on table, and mound a 3” x 3” x 13" rectangular log of filling across the length of the pastry sheet, leaving 1” on both ends
3. Roll pastry over filling. With seam-side down, cut into sections (as large or small as you like; in this video, 6 sections are made). Take each section and put on its end, and take top of dough, squeezing, pulling and twisting to form a top seal. Repeat with remaining sections
4. Dip each sealed top into egg wash and place top-up on a parchment-lined baking sheet
5. Bake 12-15 minutes, rotate sheet tray, bake 15-20 minutes longer, until puffed and golden
6. Serve with sour cream and/or mustard