Makes 16 knishes
When I first moved to New York City, I was amazed by everything. (That’s not atypical for the first year in New York City.) The smell of the streets, the faces of thousands of people coming toward me on the sidewalks, and the food. Oh, the food. Mostly, the food. When I first stepped foot into Zabar’s, a gourmet grocery store on the Upper West Side of Manhattan, I wanted one of everything. But what amazed me most was the corner in the back of the store dedicated entirely to knishes. What the heck was a knish? This soft, savory pastry, filled with a variety of fillings such as mashed potatoes and caramelized onions, seemed to be first served in street carts on the Lower East Side in the late nineteenth century. This Jewish delicatessen staple is wonderful comfort food. The original—and still the best—knish is made with schmaltz, the chicken fat skimmed off the top of homemade chicken stock. Your butcher might be able to sell you schmaltz. It’s worth seeking out as the fat for this delicacy. You could try butter or oil in place of the schmaltz, but that won’t be an authentic knish. Then again, neither is a gluten-free knish. It’s good, though.
Combine the dry ingredients. In a large bowl, whisk the flour, psyllium husks, baking powder, and salt together.
Make the dough. Make a well in the center of the dry ingredients. Add the egg, chicken fat, vinegar, and warm water. Using a rubber spatula, mix everything into a dough. The dough will be a bit shaggy and maybe feel a bit greasy to the touch. That’s okay. Make sure the dough is entirely combined. Let it sit out for 1 hour. Refrigerate the dough overnight.
Caramelize the onions. The next day, set a Dutch oven over medium-high heat. When the air above the pot feels hot to your touch, pour in the oil. Add the butter. When the butter has melted and the oil shimmers in the pot, add the onions. Toss them to coat in the oil and butter, and cook, stirring occasionally. Turn the heat down to medium. After 10 minutes, pinch the sugar and salt over the top of the onions and stir well to coat. Continue to cook the onions, stirring every 10 minutes or so, until they soften and then begin to reduce in volume. After 30 minutes of total cooking time, the onions will shrink into themselves, browning. Stir the onions again, scraping the bottom of the pot with a metal spoon, to catch all that brown caramelized goodness. Don’t let the onions burn. Continue to cook the onions, stirring and scraping the bottom of the pot every 2 to 3 minutes to make sure the onions don’t burn. After 45 minutes of total cooking time, the onions will be lovely. After 60 minutes, the onions will be dark brown, wafting a slight sweetness. Cook them as long as you wish, knowing the onions will grow darker and sweeter the longer you cook them. Turn off the heat and set aside to cool.
Cook the potatoes. Set a Dutch oven on the stovetop. Fill it three-quarters full with cold water and add enough salt to make it taste like the ocean. Add the potatoes to the cold water. Bring the water to a boil over high heat, then reduce the heat to medium. Simmer until a knife slides right through one of the potatoes without any force, 10 to 15 minutes. Drain the potatoes into a colander in the sink. Let the potatoes steam there for about 5 minutes to allow them to dry.
Rice the potatoes. If you own a potato ricer or food mill, push the potatoes through it. Or, you could push the potatoes through a fine-mesh sieve with the back of a ramekin or wooden spoon.
Mash the filling. Put the potatoes back in the Dutch oven. Fold 1½ cups of the caramelized onions and the thyme into the potatoes. (Save the rest of the onions for the tops of the knishes.)
Make the knishes. Take the knish dough out of the refrigerator. Let it sit out for 20 minutes to come to room temperature. Between two pieces of lightly greased parchment paper, roll the dough out to a large thin rectangle, about 15 x 30 inches.
Spoon the potato filling onto the top of the rectangle of dough, forming a log along the long side. Fold in the sides of the dough. Using the bottom piece of parchment paper as a push, roll the dough into a log over the potato filling, and continue rolling until you have reached the end of the dough. Seal the ends. Cut the log into 16 pieces.
Put one piece into your hand. As you can, press inward, gathering the dough toward the top of the knish to seal it. Put the knish dough side down on a parchment-lined baking sheet. Make an indentation on the top of the knish and shape the dough to make a more even circle. Repeat with the remaining knishes.
Put the baking sheet into the freezer for 30 minutes.
Prepare to bake. Preheat the oven to 450°F.
Bake the knishes. Bake the knishes for 15 minutes, then turn the baking sheet 180 degrees in the oven and bake until the tops are golden brown, about 10 more minutes.
Top each knish with more caramelized onions and serve.