Soak Panko Breadcrumbs (3/4 cup) in the Milk (1/4 cup) until soft. Pulse the Onion (1) in the food processor or use a box grater to grate it. Add to the Ground Chicken (1 pound).
Add the Egg (1), Sour Cream (2 tablespoon), Salt (1 teaspoon) and Ground Black Pepper (1/2 teaspoon).
Mix to combine. The meat mixture will be very soft. Set aside.
Chop the fresh Mushroom (8 ounce) into a small dice.
In a small bowl, place the Dried Porcini Mushroom (2 tablespoon) and cover with water. Cover with plastic wrap, poke a few holes in it and microwave for a few minutes until the mushroom are softened.
Drain and mince the mushrooms.
Mince the Shallot (1). Melt the Butter (1 tablespoon) in a skillet. Add the shallot, season with salt and pepper. Cook for about 3 minutes.
Add both of the mushrooms. Season with salt and pepper. Cook for about 7 minutes until the mushroom are softened, and all the liquid is evaporated.
Set aside the mushrooms stuffing.
Use damp/moistened hands to shape the kotleti, otherwise they will be too soft to handle. Keep moistening your hands every time you make another kotleta.
Take a portion of meat, form it into a meatball and make an indentation in the center.
Place a heaping teaspoon of mushrooms filling into the indentation.
Cover the filling with the meat on the sides and shape into an oval.
Dredge the kotleta in the All-Purpose Flour (1/2 cup), brushing off the excess.
Repeat with the remaining meat mixture and the mushrooom stuffing. You should have about 16 kotleti.
Heat Vegetable Oil (to taste) in a nonstick skillet on medium heat. Add the kotleti and cook until golden on both sides, approximately 3~5 minutes per side.
Flip and cook for another 3~5 minutes.
Remove from pan and serve!