Season the chicken cutlets with salt, pepper, garlic powder, and paprika. Sprinkle flour, and lightly rub it using your hands.
Over medium high heat, melt 1 tablespoon of butter and heat the olive oil. Working in batches, cook the chicken cutlets on both sides until cooked through. The chicken is done when the internal temperature reaches 165°F/74°C using a kitchen thermometer. Takes about 4-5 minutes per side depending on the size/thickness of the cutlets. Remove onto a plate, and set aside (keep warm).
To the same skillet, melt the 2 tablespoons of butter then add the onion and mushrooms and cook until the onion is translucent and the mushrooms are softened. Add the garlic, and cook for 30 seconds.
Add the flour, and cook for 30-60 seconds. Then add half of the chicken stock, deglaze the pan and scrape off any bits stuck to the bottom of the pan with a wooden spoon. Add the rest of the stock, sour cream, and Dijon mustard.
Use a spatula or a whisk to stir the sauce, the sour cream will look a little separated but will melt into the sauce as it cooks. The sauce will thicken in 2-3 minutes. Have a taste and adjust seasonings to your preference, season with salt and pepper. If the sauce hasn’t thickened enough, simmer for 2 more minutes.
Remove from heat, and add the chicken back to the skillet. Serve over buttered egg noodles, mashed potatoes or rice. Garnish with chopped parsley.