Cut the chicken breasts into 1-inch pieces. Season them with garlic powder, salt, and pepper, then coat them lightly in flour.
In a large skillet, heat 2 tablespoons of olive oil over medium-high heat. Cook the chicken in two batches to ensure it browns evenly. Place the chicken in the pan and cook for about 3 minutes per side, or until fully cooked (165°F internally) and lightly browned. If the pan becomes dry after the first batch, add the remaining tablespoon of olive oil. Remove the chicken from the pan and set it aside.
Add the butter to the same skillet and let it melt. Once melted, add the mushrooms and onions. Cook, stirring occasionally, for 6-8 minutes, until the mushrooms have released their moisture and browned slightly.
Stir in the Dijon mustard, Worcestershire sauce, and minced garlic. Cook for about 1 minute until fragrant.
Pour in the chicken broth, scraping the bottom of the pan to loosen any browned bits. Return the chicken to the skillet and cook for another 2 minutes, stirring occasionally.
Add the sour cream to the pan and stir to combine. Let the mixture heat through for about a minute. Be careful not to let it bubble too much to prevent curdling. Taste and adjust seasoning with more salt and pepper if needed.Add the sour cream to the pan and stir to combine. Let the mixture heat through for about a minute. Be careful not to let it bubble too much to prevent curdling. Taste and adjust seasoning with more salt and pepper if needed.
Serve the chicken stroganoff immediately and enjoy!