Cut the boneless skinless chicken breasts into 1-inch pieces and season with garlic powder, salt, and black pepper. Dredge in flour.

In a large skillet, heat olive oil over medium-high heat. Add half the chicken pieces, cooking until golden brown, about 3-4 minutes on each side. Transfer to a plate and cover to keep warm.

In the same skillet, add more olive oil if needed, along with butter. Sauté chopped onion and sliced mushrooms over medium heat for 6-8 minutes until softened and golden brown.

Stir in Dijon mustard, Worcestershire sauce, and minced garlic. Cook for about 1 minute to meld flavors.

Pour in chicken broth, scraping up browned bits. Return chicken to the skillet and let simmer quietly for 2 minutes.

Lower heat, stir in sour cream until smooth. Remove from heat before adding sour cream to avoid curdling. Cook for a couple of minutes to warm through, adjusting seasoning as needed.