Season the chicken with garlic powder, paprika, salt and pepper, then coat each piece in flour.
Heat olive oil and 1 tablespoon of the butter to a large skillet over medium-high heat. Once the pan is hot, add the chicken and cook it for about 3 minutes on each side or until cooked through (165F) & lightly browned. Transfer to a plate. Add 1 tablespoon olive oil to the skillet and cook the second batch. Transfer to the chicken on the plate.
Add 2 tablespoons butter to the pan and let it melt, then add the mushrooms and saute turning occasionally for 6-8 minutes or until their water has been released and they get a nice sear.
Add onions and saute for a couple of minutes until translucent. Stir in minced garlic. Turn the heat down to simmer.
Stir in Worcestershire sauce, Dijon mustard, and cook for about a minute.
Add in the chicken stock and scrape up any brown bits from the bottom of the pan. Add 2 Tbsp of the chicken stock to a small bowl with 2 Tbsp of flour (or 1 Tbsp cornstarch), and mix well making a slurry.
Stir in the sour cream and the slurry into the skillet. Let it simmer stirring occasionally on medium-low heat for about 3 minutes until it thickens.
Add the chicken back in and cook until it’s warmed through, stirring occasionally.
Season with more salt & pepper as needed and serve immediately.
Video
Notes
Cook the chicken through when you cook it in the skillet. If you have an instant-read thermometer, it should register at 165F.
Don't boil the stroganoff sauce. If you do, the sour cream can curdle.
You can also make this recipe with boneless skinless chicken thighs.
Don't cover the skillet when you cook the mushrooms. Let them brown undisturbed for a few minutes on each side until their liquid has evaporated and they have a nice brown sear.