Place potatoes into a large pot and cover with salted water. Bring to a boil, reduce heat, and simmer until tender (about 25 minutes). Drain and mash the potatoes. Season with salt and pepper.

Combine flour and 1 teaspoon salt in a large bowl. Mix sour cream with baking soda in a separate bowl. Add water to the flour mixture, then mix in the sour cream mixture. Knead into a smooth dough, cover with plastic wrap, and refrigerate for 10-15 minutes.

Melt butter in a skillet over medium heat. Cook onions until browned (5-10 minutes), then add mushrooms and cook for another 5 minutes. Combine the onion-mushroom mixture with the mashed potatoes.

Divide dough into 4 portions. Roll each into a 1.5-inch rope and cut into 0.5-inch pieces. Flatten pieces into circles, fill with potato mix, and seal into half-moon shapes.