(I do not aim to copy the recipe here. Can be found in Mastering the Art of French Cooking)
In this first (blurry) picture, I have just added the liquid (1 cup dry white wine + 1.5 cup chicken stock) to the chicken and butter-braised onions & carrots. About 5 sprigs each of thyme and parsley are your herbs (you can also add a bay leaf):
Once the chicken is done, you remove it from the pan / casserole and place in a buttered (optional) glass dish. I covered it with foil but you do not have to keep it warm at this stage since the last step in the dish will heat it up.
Pour all the liquid through a sieve into a stainless steel pot. Heat up to boil and skim any foam off. Keep boiling while making the roux and velouté.
Create a 'roux'
In a seperate pot, melt 1.75oz of butter until bubbling but not browning. Add 1oz flour and stir well. Let it bubble for about 2 minute, stirring continually (lift pot from heat if it threatens to brown the roux.
Make the 'velouté '
Pour all the boiling stock into the roux and whisk vigorously with a whisk (remember to use a silicone one if you have a non-stick pot!). Place over low heat and keep whisking as the sauce thickens.
Turn your 'velouté' into a Sauce Mornay
Add about 1.5 oz of coarsely grated Gruyère cheese to the sauce and stir until well mixed. Pour sauce over the chicken.
Sprinkle another 1.5 oz (or more to taste) of cheese over the top and place under a hot grill (if the chicken was still warm). If the chicken was cold you need to put it into an oven, preheated to 375F for 25 - 30 minutes until the dish is bubbling nicely. (Careful not to re-cook the dish as it will dry out the chicken - especially if you have white meat pieces).