Fry the chicken strips in a pan over medium-high heat for 5-7 minutes until the internal temperature reaches 74°C / 165°F. Cut the leek into rings, add to the pan, and fry for 2 minutes more. Add the water to deglaze the pan.
Add the crème fraîche and reduce heat to medium. Simmer for 15 minutes, stirring every 2-3 minutes. Season with the vegetable stock, salt, and pepper. If needed, stir in the sauce thickener to reach desired consistency. Stir in the parsley.