Step 1

To make the dough: Combine cold seltzer, apple cider vinegar and salt in a small bowl.

Step 2

Place flour and baking powder in a food processor and add seltzer mixture. While pulsing, drizzle olive oil until mixture starts to come together.

Step 3

Remove mixture from food processor and place on a lightly floured surface. Knead using the palm of your hands until dough becomes smooth and elastic, about 3 minutes.

Step 4

Wrap in plastic and let rest in the fridge for 1 hour.

Step 5

To make the filling: Peel and cut 1 Idaho potato into large chunks around the same size. Bring a pot of salted water to a boil and cook potatoes for 7-10 minutes, or until fork tender. Drain and allow to cool slightly. When potatoes have cooled a few minutes, mash.

Step 6

Finely chop one small onion. Heat a few Tbsp olive oil or schmaltz in a pan. Saute onions until translucent, about 5-8 minutes. Add a pinch of salt and pepper.

Step 7

Combine onions and mashed potatoes and 1 Tbsp olive oil. Add pinch more salt if desired (remember – potatoes need a lot of seasoning).

Step 8

To assemble: Preheat oven to 375°F.

Step 9

After dough has rested, roll out dough very thin (as thin as you can get) on a lightly floured surface. This is definitely the trickiest part as the dough wants to bounce back so keep at it. Using a large cookie cutter or cake round (4-7 inches ideal) cut out rounds. Depending on the size, fill generously with potato-onion mixture.

Step 10

Fold up sides on top of dough and pinch tightly. Beat 1 egg and brush all over each knish.

Step 11

Bake for 20-25 minutes, or until just golden.

Step 12

Serve with mustard or other favorite dipping sauce.