Chicken and leek in white wine sauce

It still bemuses me that I can't thicken sauces with cornflour to save my life, yet I can casually, and with no thought at all, make a perfect roux every time. So, it must be easier than you think - give it a go!

Ingredients
1lb chicken thighs, skinned
1tsp chopped garlic
1tsp dried tarragon
Splash rapeseed oil
375ml white wine (Visionario Binaco Delle Venezie 2008)
1 large onion, diced
2 large leeks
1 1/2 oz butter (unsalted)
1 1/2 oz plain flour
100ml semi-skimmed milk

Method
Heat oil, garlic, onion and tarragon. Sear chicken thighs. I used a large frying pan for this stage, then turn down heat, decant into saucepan, add 250ml wine and chopped leeks. Simmer for 30-45 mins (in my case, while roasting squash, parsnip and carrot to go with it) and make white sauce.

Melt butter over a low heat, then stir in the flour until it forms a paste coming cleanly away from the sides of the pan. I used the frying pan, thereby deglazing it at the same time. When you have your paste, remove from the heat, add 100ml cold wine, and beat until smooth. I randomly added a further 150ml or so of cold milk, beat until smooth. Next I strained the wine stock from the saucepan with the chicken and gradually added this to the sauce. When it was all incorporated, I poured it back over the chicken and leek mixture and simmered for a further 5-10 mins or so. You can add more wine (or milk, or water) if it gets to thick during this stage, but do it carefully!