Cut the chicken into 1" pieces. Sprinkle them with garlic powder and generously with salt & pepper, then coat each piece in flour.

Add 2 tablespoons of the oil to a deep skillet over medium-high heat. Cook the chicken in 2 batches so the pan isn't over-crowded and the chicken browns evenly. Cook the chicken for about 3 minutes per side or until cooked through (165F) & lightly browned. For the second batch, add another tablespoon of olive oil if the pan goes dry. Transfer cooked chicken to a plate.

Add the butter to the pan and let it melt, then add the mushrooms and onions. Sauté for 6-8 minutes or until their water has been released and the mushrooms get a nice sear.

Stir in the Dijon mustard, Worcestershire sauce, and garlic. Cook for about a minute.

Add in the broth and scrape up any browned bits from the bottom of the pan. Add the chicken back in and cook for 2 minutes, stirring occasionally.

Stir in the sour cream and let it heat through for about a minute (don't let it bubble too much or there's a chance it'll curdle). Season with more salt & pepper as needed and serve immediately.