Heat the oil in a large saucepan, add the onion, garlic and leeks and fry for 3–4 minutes. Transfer the vegetables to a plate, add the chicken to the pan and fry for 3 minutes until beginning to colour all over.
Add the wine and simmer until it has reduced by about half. Add the flour and cook, stirring, for 1 minute, then gradually add the stock and cook, stirring all the time, until the sauce has thickened.
Stir in the leek mixture, cream, tarragon and mustard and season well. Transfer to an ovenproof dish, then sprinkle over the breadcrumbs and Gruyère.
Place in a preheated oven, 200°C (400°F), Gas Mark 6, for 25–30 minutes until golden and bubbling. Serve with steamed vegetables.