Directions

Make Dough: 

Combine dry dough ingredients in large mixing bowl, and wet dough ingredients in medium bowl. Make well in dry ingredients. Pour wet ingredients into center; stir to combine until dough ball forms.  

Transfer dough to clean work surface; knead about 1-2 minutes to fully incorporate all flour. Return dough to bowl; cover with plastic wrap and let rest ½ – 1 hour at room temp. 

Make Filling: 

In large skillet, combine butter/schmaltz and olive oil over medium heat. Once melted, add onion, thyme, bay leaf. Season with salt/pepper. Reduce heat to medium low and stir occasionally until onions are golden/caramelized, about 50 minutes. Remove from heat and take out bay leaf and thyme sprigs. 

To large pot, add steamer basket and water to just below basket level. Bring water to boil; add potatoes and cover with lid. Cook about 15-20 minutes, until potatoes are tender. Remove steamer basket and drain water. Transfer potatoes back to pot, add sour cream. Mash until fairly smooth (some lumps are fine). Stir caramelized onions into potatoes and salt/pepper to taste. 

Assemble: 

Preheat oven to 375°, line a baking sheet with parchment paper.  Divide dough in half, place one half back under plastic wrap. Re-knead to re-incorporate any oil that may have come out of dough while it rested.  

Shape half of dough into a rectangle and roll it out into a large rectangle (about 1/8” thick, 6” wide and 12” long – or longer if possible). The dough’s not particularly sticky, but if you need to lightly flour the work area, go ahead. 

Shape half the filling into a 2”-wide log. Lay it on rolled-out dough, about 1-2 inches from bottom edge. Lift this 1-2 inches of dough up/over filling log, roll it away from you until you get to the other edge of the dough…like rolling sushi. Adjust the dough/filling log so the seam is on the bottom and filling evenly distributed. 

Trim dough ends even with filling. Lightly indent dough/filling log with the dull side of a knife about every 2 inches so you have six equal pieces; use sharp side of the knife to cut pieces.  

Gently press filling inward, stretch dough over filling on one end and pinch to seal. Set knish with this end down on a clean work surface. Press cut dough in towards center on the upward-facing end; you’ll have a slight indent with some filling showing.  

Repeat process with remaining knishes and then entire process with second half of dough. 

Place knishes on prepared baking sheet and brush with egg/water mixture. Bake until golden, about 45-50 minutes, turning pan halfway through. Let cool ½ hour before serving. 

Note: Dough can be left in fridge 3 days before filling/baking, bring to room temp before using.