Heat butter in a large skillet over medium-high heat and brown minced chicken until no longer pink; drain excess liquid if needed.
Add onion and mushrooms; cook 5–7 minutes until softened. Stir in garlic for 30 seconds.
Sprinkle flour over the mixture and cook 1 minute, stirring to remove raw taste.
Slowly pour in chicken broth, scraping browned bits, and simmer until slightly thickened.
Reduce heat and stir in sour cream and Dijon; warm through without boiling. Season with salt and pepper.
Serve over noodles or mashed potatoes and garnish with parsley.