Season the chunks of chicken with salt and pepper. Dredge them in flour, shaking off any excess.

Heat olive oil and 1 tablespoon of butter in a large skillet over medium-high heat. Sear chicken for 3-4 minutes on each side until golden brown. Remove and set aside.

In the same skillet, add 2 tablespoons of butter and let it melt. Sauté the chopped onion until translucent, then add minced garlic and cook for 30 seconds until fragrant.

Add the sliced mushrooms to the skillet and cook for about 6-8 minutes until browned and moisture evaporates.

Sprinkle 1 tablespoon of flour over the cooked mushrooms. Stir well and cook for 1 minute, then gradually whisk in chicken broth and Dijon mustard. Let simmer for a couple of minutes.

Lower the heat and stir in sour cream until the sauce is smooth. Return seared chicken to the skillet and simmer for an additional 5 minutes.

Adjust seasoning with salt and pepper. Garnish with chopped parsley and serve hot over mashed potatoes, pasta, or rice.