For the pelmeni: Put the flour, milk, oil, egg, 1/2 cup water and 1/2 teaspoon salt together in stand mixer bowl and mix on medium-high speed until dough starts pulling from the side. Take it out of the bowl and place on a floured surface. Knead by hand for 2 to 3 minutes and place back in bowl. Cover with plastic wrap and rest for 30 minutes before using. Divide into 2 balls of dough.
Mix together ground chicken and onion in a bowl and add some salt and pepper. Sprinkle flour on a pelmeni mold and roll out first ball of dough to fit the mold. Drape on top of the mold and press lightly. Fill the holes with the chicken mixture. Roll out the second ball and drape on top. Trim the edges. With a large rolling pin, start rolling gently at first and then more firmly until the shape cuts through. Turn the mold around and tap out the pelmenis.
For the soup: Add the stock, tomatoes, mushrooms and half the dill to a small pot and bring to a boil. Add the pelmenis and let boil until they float on top, 5 to 7 minutes. Divide soup among 4 bowls and add dollops of sour cream and the rest of the dill.
This recipe was provided by a chef, restaurant or culinary professional. It has not been tested for home use.