Step 2

Mix together the flour, baking soda, psyllium husk and salt. In a separate bowl, whisk together the egg white, schmaltz, apple cider vinegar and water.

Step 3

Combine the wet and dry ingredients and knead into a dough. It should be kneadable, but slightly tacky. Place in a small bowl and cover with plastic wrap. Set aside and let rest for 45 minutes to 1 hour.

Step 4

Meanwhile, in a medium saucepan, heat 1 tablespoon (15 ml) of the schmaltz or olive oil over medium heat and add the sliced onions. Turn the heat down to medium-low and sauté until deeply brown and very soft, about 45 minutes.

Step 5

Peel and slice the potatoes into 1-inch (2.5-cm) slices and place in a medium pot and cover with water and a pinch of salt. Bring to a boil and cook until soft, about 20 minutes.

Step 6

Drain the potatoes and allow them to cool slightly. Once the onions are caramelized, mash them in with the potatoes, along with the remaining schmaltz or olive oil and the salt. (For a spinach knish add the spinach now.)

Step 7

Preheat the oven to 375°F (190°C) and line a baking sheet with parchment paper.

Step 8

Divide the dough into two halves and roll one half out on a sheet of parchment paper until it’s a rectangle about 12 x 9 inch (30 x 23 cm). Cut the dough into four sections. Place about ¼ cup (60 ml) of the filling in the center of each rectangle, leaving a half-inch (13-mm) border around the outside. Roll out the remaining dough and cut into four rectangles. Use these to make the tops of the knishes, pressing the ends together and trimming off the extra dough on the outside.

Step 9

Whisk the egg yolk with a teaspoon (5 ml) of water and brush the knishes with the egg wash.

Step 10

Transfer the knishes to the prepared baking sheet and brush with the additional beaten egg. Bake for 12 minutes and then flip and bake for an additional 6–8 minutes.

Step 11

Serve with mustard.  Recipe posted with permission from The New Yiddish Kitchen: Gluten-Free and Paleo Kosher Recipes for the Holidays and Every Day , by Simone Miller & Jennifer Robins