Potato and Mushroom Vareniki

Start by submerging potatoes into a substantial container filled with salted water; heat it up until boiling point. Lower the temperature to a moderate level and let simmer for 25 minutes, or until they're tender. Remove potatoes from the water with a strainer and mash them; add some salt and pepper to taste.

While potatoes are cooking, mix together flour and a pinch of salt in a large bowl. In another bowl, whisk sour cream and baking soda together; then gradually add water to the flour mixture and blend in the sour cream mixture. Combine both mixtures until you have a smooth dough, cover it with plastic wrap, and refrigerate for 10 to 15 minutes.

Meanwhile, melt butter in a skillet over medium heat and cook sliced onions until they turn golden brown. Add sliced mushrooms to the skillet and continue cooking until they're also browned, around 5 minutes. Add the cooked onions and mushrooms to your mashed potatoes.

Divide the dough into four equal parts and roll each part into a 1 1/2-inch thick rope. Cut the ropes into small, 1/2-inch pieces and press each piece into a flat circle. Place one teaspoon of your potato filling in the center of each dough circle and fold the dough over into a half-moon shape, sealing the edges.

Prepare another large container of salted water and bring it to a gentle boil. Carefully add the vareniki in batches, making sure they're fully submerged in water. Cook until they float to the surface, about 3 minutes. Continue cooking for an additional 2-3 minutes or until they're cooked through. Remove the vareniki with a slotted spoon and place them in a colander to drain off excess water.