Cut the chicken breasts into uniform 1-inch pieces. Season generously with garlic powder, salt, and black pepper, then lightly coat each piece in flour by pressing them into a thin layer spread on a plate. Shake off any excess flour.

Heat 2 tablespoons of olive oil in a large, deep skillet over medium-high heat until shimmering. Working in batches to avoid crowding, sear the chicken pieces for about 3 minutes per side until golden brown and just cooked through. Transfer the seared chicken to a plate and set aside with any accumulated juices.

Reduce the heat to medium and add the remaining tablespoon of olive oil along with the butter to the same skillet. Once the butter melts, add the sliced mushrooms and chopped onion. Sauté for 6 to 8 minutes, stirring occasionally, until the onions turn translucent and the mushrooms develop a deep brown color.

Add the minced garlic, Dijon mustard, and Worcestershire sauce to the vegetables. Stir constantly for about 1 minute until fragrant. Pour in the chicken broth and use a wooden spoon to scrape up all the browned bits stuck to the bottom of the pan.

Return the seared chicken and any collected juices to the skillet. Let everything simmer together for 2 minutes, allowing the chicken to finish cooking and the flavors to blend.

Turn the heat down to low. Stir in the sour cream and gently heat through for about 1 minute, just until the sauce is warmed and creamy. Do not let the sauce boil after adding the sour cream or it will curdle.

Taste and adjust seasoning with additional salt, pepper, Dijon, or Worcestershire if needed. Garnish with chopped fresh parsley or sliced green onions and serve immediately over egg noodles, rice, or mashed potatoes.