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

2. Heat 2 tablespoons of olive oil in a deep 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. Transfer the cooked chicken to a plate. Repeat with the remaining chicken, adding more oil if needed.

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

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

5. Pour in the chicken broth, scraping up any browned bits from the bottom of the pan. Return the cooked chicken to the skillet and simmer for about 2 minutes, stirring occasionally.

6. Reduce the heat to low and stir in the sour cream until the sauce is smooth. Let it heat through for about a minute without boiling. Taste and adjust seasoning with salt and pepper before serving.