Cut the boneless skinless chicken breasts into 1-inch pieces. Season with garlic powder, salt, and black pepper, then coat with flour.

Heat 2 tablespoons of olive oil in a deep skillet over medium-high heat. Add half of the chicken and cook for about 3 minutes per side until golden brown and fully cooked. Transfer to a plate.

Repeat the sautéing process with the remaining chicken in the same skillet. Transfer to the plate with the first batch.

Lower the heat, add remaining olive oil and butter. Sauté chopped onion and sliced mushrooms for 6-8 minutes until browned.

Stir in Dijon mustard and Worcestershire sauce. Optionally add minced garlic and cook for another minute.

Pour in chicken broth, scraping up browned bits. Simmer and return chicken to the skillet for about 2 minutes.

Reduce heat to low, stir in sour cream until smooth. Adjust seasoning if necessary before serving.