Warm the oil in a two-to-three-quart pot. Chop the onion and sauté until lightly golden, about 15 minutes.

Pour the oat milk into the pot, season with salt, pepper, and garlic powder and bring to a simmer. Add in the mashed potato flakes and stir. Remove from heat and continue to stir until smooth.

Mix in mustard. Slowly pour in hot water until smooth and creamy. (You may not need all of the water).

Allow the mixture to cool. This can be made ahead of time and stored in the refrigerator. 

Preheat the oven to 375 degrees Fahrenheit.

Lay out dough on a piece of Gefen Parchment Paper. Dough should be cold but not frozen. Spread a line of mashed potato down the center of each square.

Using a sharp knife, make horizontal slits on each side of the potato filling. “Braid” the strips of dough to cover the filling. Optionally, whisk an egg with a teaspoon of oat milk and brush over each knish.

Bake for approximately 20 minutes. Cool slightly before transferring to a serving platter.