Cut the chicken breast halves into 1-inch pieces. Sprinkle them evenly with garlic powder, salt, and black pepper to taste. Coat each chicken piece in flour, ensuring all sides are lightly covered.

Heat 2 tablespoons of olive oil in a deep 10-12 inch skillet over medium-high heat. When hot, add half of the coated chicken pieces, making sure not to overcrowd the pan. Cook for about 3 minutes on each side until golden brown and cooked through (165°F internal temperature). Transfer the cooked chicken to a plate. Repeat with remaining chicken.

In the same skillet, reduce heat to medium and add 1 tablespoon olive oil and the butter. Once melted, add the sliced cremini mushrooms and chopped onion. Cook for 6-8 minutes, stirring occasionally, until the vegetables release their water, it evaporates, and the mushrooms are deeply browned and seared.

Stir in the Dijon mustard, Worcestershire sauce, and minced garlic. Cook for about a minute, stirring constantly, until the mixture is fragrant.

Pour in the chicken broth, using a wooden spoon to scrape up any browned bits stuck to the bottom of the pan. Return the browned chicken to the skillet and simmer for about 2 minutes, stirring occasionally, so flavors meld and the chicken warms through.

Reduce the heat to low and stir in the room temperature sour cream until the sauce is smooth. Let it heat through for about a minute, but don't allow the mixture to boil to prevent curdling. Taste the sauce and adjust with salt and pepper as needed before serving.