Melt butter in a small saucepan over medium heat. Add flour. Stir to create a paste. Cook for 1 minute. Whisk in milk until mixture forms a smooth and thick paste. Gradually whisk in stock, onion powder, garlic powder, thyme and salt until mixture is smooth. Cook, whisking constantly, for 2 to 3 minutes or until mixture thickens. Remove from heat. Set aside to cool 10 minutes.
Preheat the oven on high while you prepare the remaining of the ingredients.
Meanwhile, heat oil in a oven proof skillet over high heat. Add chicken, Cook for 3 minutes. Turn. Cook for 2 minutes. Transfer to a plate. Preferably shocked not to be completely cooked as there will be further cooking time.
Return to heat. Add garlic and onion & mushrooms. Cook, stirring, for 2 minutes. Add chicken stock allow to Simmer, stirring, for 3 minutes. Add In the chicken paste ,stock, mustard and sugar. Stir to dissolve mustard. Add carrot, potato, rosemary and thyme. Stir to combine.
Return chicken to pan. Cover. Transfer to oven. Bake for 30– 40minutes. Remove lid. Bake for a further 8-10 minutes or until chicken is dark brown and cooked through. Stand 5 minutes. Serve sprinkled with extra thyme.