Begin by cutting the chicken breast halves into 1-inch pieces. Season them generously with garlic powder, salt, and black pepper before lightly coating each piece in flour.

Heat a large skillet over medium-high heat and drizzle in olive oil. Add the chicken pieces in batches, cooking for about 3 minutes per side until golden brown and cooked through.

In the same skillet, reduce the heat to medium. Add butter with a bit more olive oil if needed, then toss in sliced cremini mushrooms and chopped onions, stirring frequently for 6–8 minutes until browned and translucent.

Stir in Dijon mustard, Worcestershire sauce, and minced garlic into the vegetables and cook for about 1 minute to soften the garlic.

Pour in the chicken broth, scraping the skillet to release any browned bits. Return the cooked chicken to the skillet and stir to combine, allowing to simmer together for about 2 minutes.

Lower the heat and stir in the full-fat sour cream until the sauce is creamy and smooth, tasting and adjusting seasoning as necessary.