Melt butter and oil in a large pan over medium heat.
Sauté the onion until soft and translucent.
Add peppers and mushrooms, and cook until softened and fragrant.
Stir in the flour and cook for 1–2 minutes to form a roux.
Gradually add chicken stock while stirring to prevent lumps.
Add milk or cream and continue stirring until the sauce thickens.
Season with salt, pepper, Worcestershire sauce, and paprika.
Add the cooked chicken and simmer gently for 5–10 minutes to heat through.
Garnish with chopped parsley before serving.
Serve over fluffy rice, pasta, or buttered toast.