2 pounds Idaho® russet potatoes, peeled and quartered (4 medium)
2 tablespoons cooking oil, such as canola
2 tablespoons butter or schmaltz (chicken fat)
3 medium yellow onions, peeled and diced (1 1/2 pounds)
2 large eggs
1/3 cup matzo meal or all purpose flour
5 ounces spinach leaves, slivered or roughly chopped
Salt and Pepper
1/2 cup sour cream
1/4 cup finely minced red onion, optional
Preheat oven to 375ºF and set oven racks in the top and bottom of the oven.
Place potatoes and 2 teaspoons salt in a pot and cover with cold water by an inch. Bring to a boil, then gently boil for 20 minutes or until potatoes are tender. Drain, mash well, and set aside to cool.
Heat oil and butter (or schmaltz) over medium heat in a large skillet. Cook onions, stirring occasionally, for 20 minutes, to brown. Scrape onions into the bowl of a food processor. (Don’t clean the skillet – you’ll use it for the spinach.) Add eggs, 1 teaspoon salt, 1/4 teaspoon pepper to the onions and pulse several times until the onions are finely chopped. Scrape the onion-egg mixture into the bowl with the potatoes. Add matzoh meal. Mix thoroughly.
Add the chopped spinach to the skillet and sauté over medium heat, stirring constantly, until wilted (a minute or two) Add to the potato mixture and stir through. The dough is now ready to make into knishes or, if you’re prepping ahead, roll into a ball, cover tightly with plastic wrap, and store in the fridge for up to 24 hours. If you chill the dough, let it sit out for 30 minutes before proceeding to the next step.
Spray or lightly oil two baking sheet pans. Scoop up a heaping tablespoon of potato mixture, roll it into a ping-pong sized ball, place it on the prepared pan. Repeat until you’ve used up all the potato mixture. You should get about 50 pieces. Push your thumb down on the center of each ball to flatten it slightly and make a depression – similar to how you’d make thumbprint cookies.
Bake knishes for 30 minutes in the top and bottom of the oven, switching the position of the trays half way through. Turn the oven up to broil and let the top tray brown for 2-3 minutes. Remove it and set the other tray on the top shelf and let it brown for 2-3 minutes.
Set knishes out on a platter or serving tray. Dollop a little sour cream in the center of each knish and serve.