Cut the chicken into 1-inch pieces. Sprinkle with garlic powder and season generously with salt and pepper. Coat each piece in flour.
Heat 2 tablespoons of olive oil in a deep skillet over medium-high heat. Cook the chicken in batches, ensuring the pan isn’t overcrowded. Brown the chicken for about 3 minutes per side, flipping once, until cooked through (165°F) and lightly browned. Adjust heat as necessary for the second batch, adding more oil if needed. Transfer cooked chicken to a plate.
In the same pan, melt the butter and add the mushrooms and onions. Sauté for 6-8 minutes, until the mushrooms release their water, it evaporates, and the mushrooms are nicely seared.
Stir in the Dijon mustard, Worcestershire sauce, and garlic. Cook for about a minute.
Pour in the chicken broth, scraping up any browned bits from the bottom of the pan. Return the chicken to the pan and cook for an additional 2 minutes, stirring occasionally.
Stir in the sour cream and heat through for about a minute, being careful not to let it bubble too much to prevent curdling. Adjust seasoning with more salt and pepper if needed. Serve immediately.