Start by cutting the chicken breast halves into 1-inch pieces to ensure even cooking. Season with garlic powder, salt, and black pepper, then coat lightly in flour. Set aside.

Heat a large skillet over medium-high heat and add olive oil. Add seasoned chicken pieces in batches and cook for about 3 minutes on each side, or until golden brown and reaching an internal temperature of 165°F. Remove chicken and set aside.

In the same skillet, reduce heat to medium and add butter with a splash more olive oil if needed. Toss in sliced cremini mushrooms and chopped onions, sautéing for approximately 6-8 minutes until mushrooms are browned and moisture evaporated.

Stir in Dijon mustard, Worcestershire sauce, and minced garlic to the vegetables in the skillet. Cook for about 1 minute until fragrant, stirring frequently.

Pour in chicken broth, scraping up any browned bits stuck to the bottom. Return the cooked chicken and let it simmer together for 2 minutes.

Lower heat and stir in sour cream, mixing until creamy and heated through, being careful to avoid boiling.