Boil the potatoes in salted water for about 20 minutes until fork-tender. Drain completely.

While potatoes boil, heat two tablespoons olive oil in a sauté pan over medium-low heat. Add all the diced onions and sauté until deeply caramelized and golden brown. Set aside half of these onions to fold into your dough later.

Add the chopped mushrooms to the remaining onions in the pan and cook until browned. Stir in the minced garlic. Increase heat to medium-high, pour in the Marsala wine, and simmer until reduced by half. Whisk one tablespoon of potato starch into the chicken stock, pour it into the pan, and boil until the mixture is thick. Stir in the parsley, 1/2 teaspoon salt, and 1/2 teaspoon pepper. Set aside to cool.

Mash the warm potatoes until smooth. Mix in the three eggs, two tablespoons olive oil, two teaspoons salt, 1/2 teaspoon pepper, and the reserved caramelized onions. Gradually mix in the Gefen Potato Starch (start with one cup and add the extra 1/4 cup only if needed). The dough should be soft and moldable, like pliable clay.

Preheat your oven to 400 degrees Fahrenheit and line a baking sheet with Gefen Parchment Paper. Take a palmful of dough and flatten it slightly in your hand. Press a well into the center, add one teaspoon of mushroom filling, and pull the edges up to close. If needed, use a small amount of extra dough to seal the top.

Beat the egg yolk. Using a pastry brush, brush the yolk onto the knish.

Bake at 400 degrees Fahrenheit for 30 minutes until the tops are golden.