Cut chicken breast halves into 1-inch pieces. Sprinkle evenly with garlic powder, salt, and black pepper. Coat each piece in flour, ensuring all sides are lightly covered.
Heat 2 tablespoons olive oil in deep skillet over medium-high heat. Add half the chicken (from 1 breast) without crowding. Cook 3 minutes per side until golden brown and internal temperature reaches 165°F with juices running clear. Transfer to plate. Add 1 tablespoon oil and cook remaining chicken. Set aside.
Reduce heat to medium. Add remaining 1 tablespoon olive oil and butter to same skillet. Once butter melts and stops foaming, add mushrooms and onion. Cook 6-8 minutes, stirring occasionally, until vegetables release water, it evaporates, and mushrooms develop golden brown edges.
Stir in Dijon mustard, Worcestershire sauce, and minced garlic. Cook about 1 minute, stirring constantly, until fragrant. Watch carefully so garlic doesn’t burn.
Pour in chicken broth, scraping up browned bits from bottom with wooden spoon. Return chicken to skillet and simmer 2 minutes, stirring occasionally, until flavors meld and chicken warms through.
Reduce heat to low. Stir in room temperature sour cream until sauce is smooth. Heat through about 1 minute without boiling to prevent curdling. Sauce should coat the back of a spoon. Taste and adjust salt and pepper as needed before serving.
Notes
Use full-fat sour cream at room temperature for creamiest results and to prevent curdling.
Coat chicken in flour – it helps with browning and naturally thickens the sauce.
Cook mushrooms until they release moisture and turn golden for best flavor.
Cut chicken into even-sized pieces for uniform cooking.
Don’t let sauce boil after adding sour cream to prevent separation.
For make-ahead: Brown chicken and cook mushrooms up to 1 day ahead, store separately, then combine and add sour cream when ready to serve.