Cut the chicken breast halves into 1-inch pieces. Season evenly with garlic powder, salt, and black pepper. Lightly coat each piece in flour on all sides, shaking off any excess.

Heat 2 tablespoons of olive oil in a deep skillet over medium-high heat. Add half the chicken in a single layer without crowding the pan. Cook for about 3 minutes per side until deep golden brown and cooked through to 165 degrees F internal temperature. Transfer to a plate. Repeat with remaining chicken, adding more oil if the pan looks dry.

Reduce heat to medium. Add 1 tablespoon olive oil and the butter to the same skillet. Once the butter melts and starts to foam, add the mushrooms and onion. Cook for 6 to 8 minutes, stirring occasionally, until the liquid from the vegetables cooks off and the mushrooms pick up some golden color.

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

Pour in the chicken broth and scrape up any browned bits from the bottom of the pan. Return the seared chicken to the skillet and simmer for about 2 minutes, stirring occasionally, until everything is warmed through and the sauce starts to come together.

Reduce heat to low. Stir in the sour cream until the sauce is smooth and creamy. Heat through gently for about 1 minute. Do not let it boil or the sour cream may curdle. Taste and adjust salt and pepper before serving.