Season the chicken pieces with garlic powder, salt, and black pepper. Coat each piece lightly in all-purpose flour on all sides to build a golden crust and help thicken the sauce.

Heat 2 tablespoons of olive oil in a deep skillet over medium-high heat. Cook the chicken in two batches, about 3 minutes per side, until golden brown and cooked through to an internal temperature of 165 degrees F. Avoid overcrowding the pan. Transfer to a plate and set aside.

In the same skillet, reduce heat to medium. Add 1 tablespoon olive oil and the butter. Once melted, add the sliced mushrooms and chopped onion. Cook for 6 to 8 minutes, stirring occasionally, until moisture evaporates and mushrooms are golden brown.

Stir in the Dijon mustard, Worcestershire sauce, and minced garlic. Cook for 1 minute, stirring constantly, until fragrant.

Pour in the chicken broth and scrape up any browned bits from the bottom of the pan with a wooden spoon. Return the cooked chicken to the skillet and simmer for 2 minutes, stirring occasionally.

Reduce heat to low and stir in the sour cream until the sauce is smooth. Heat gently for 1 minute but do not boil. Taste and adjust salt and pepper before serving.