Cut the chicken breasts into 1-inch pieces. Season with garlic powder, salt, and pepper. Lightly dredge each piece in flour until evenly coated.
Heat 2 tablespoons of olive oil in a deep skillet over medium-high heat. Add the chicken in batches and cook for about 3 minutes per side, flipping once, until the chicken is cooked through and lightly browned. Remove the chicken from the skillet and set aside.
In the same skillet, melt the butter, then add the sliced mushrooms and chopped onions. Sauté for 6-8 minutes until the mushrooms release their moisture and begin to brown.
Stir in the Dijon mustard, Worcestershire sauce, and minced garlic. Cook for another minute to release the flavors.
Pour in the chicken broth and scrape up any browned bits from the bottom of the pan. Add the cooked chicken back into the skillet and let it cook for about 2 minutes, stirring occasionally.
Stir in the sour cream, heating it through gently for about a minute. Be careful not to let the sauce boil, as it could cause the sour cream to curdle. Taste and adjust seasoning with more salt and pepper, if necessary.
Serve the creamy chicken and mushrooms immediately with your favorite sides.
