Makes 25 to 30, serves 5 to 6

Dumplings
2	C	Fresh, full-fat plain yogurt, preferably
1		Egg, lightly beaten
1	tsp	Salt
2 1/4	C	All purpose flour

Filling
1	Very lg.	Potato (1/2 to 3/4 pound)
4	tbsp (1/2 stick)	Unsalted butter
1		Onion, chopped
1	lb	White mushrooms, trimmed and minced
		Salt and freshly ground black pepper, to taste
		Fried onions or sour cream, for serving

1. Make the dumpling dough: Lightly beat the yogurt, egg and salt together with an electric mixer. Slowly add the flour and continue beating until smooth. Knead the dough on a floured surface until smooth. Add water, a teaspoon at a time, only if needed to smooth out the dough; it should feel like pizza dough. Form the dough into a ball, wrap it in a kitchen towel, and set aside in a cool place for 2 hours.

2. Meanwhile, make the filling: Peel and quarter the potato. Cover with cold water, bring to a boil, lower the heat, and simmer until it offers no resistance when pricked with a fork, 15 to 20 minutes. Drain and set aside.

3. Melt 2 tablespoons butter in a skillet over very low heat. Whip the potato with the butter until it is fluffy and creamy. Set aside.

4. Melt the remaining 2 tablespoons butter in the skillet over medium heat. Add the onion and saute until translucent, 5 minutes. Add the mushrooms, salt, and pepper and saute until the mushrooms are tender and any liquid exuded by the vegetables evaporates, about 5 minutes. Remove the mixture from the skillet and set aside to cool slightly. Mix the vegetables into the mashed potatoes.

5. Bring a large pot of salted water to a boil.

6. Meanwhile, roll out the dough 1/8 inch thick. Cut into 3-inch rounds with a cookie cutter or the rim of a glass. Put about 1 heaping teaspon of the mashed potato filling on one side of the circle. Fold the other side over the filling so the pierogi looks like a half moon. Press the edges together with the thumb and index finger, sealing the pierogi.

7. Lower the heat so the boiling water simmers gently. One at a time, add the pierogis to the water in batches of 4 to 6. Simmer until cooked through, 5 to 7 minutes. Remove with a slotted spoon and drain on paper towels. Keep warm while cooking the remaining pierogis. Serve with fried onions or sour cream.