This is a rather posh description of a dish using up leftovers. I apologise for the vagueness of the quantities, my only excuse being that it's in the spirit of an improvised recipe. For 4.
4 portions cold chicken
About 56g butter
I clove garlic, chopped
About 600ml chicken stock
2tbsp creme fraiche or double cream
1tsp Dijon mustard
Handful flat-leaf parsley, chopped
Salt and pepper
Arrange the chicken in a gratin dish.
Melt the butter over a gentle heat in a heavy-bottomed saucepan. Soften the garlic in it for a minute or so, and then stir in the flour, allowing it to cook for a minute. Add more butter if the roux seems thick: it should be the consistency of loose, wet sand. (My general rule in recipes involving a roux-based sauce is to allow as much as will be thickened by a dstsp of flour for each person.) It does not matter if this roux -- unlike the one for a bechamel -- takes on a little colour.
Turn up the heat and add the stock, portion by portion, incorporating each one until the lumps disappear before adding the next. Stop when you have a pourable, coating consistency, not too thick. Stir in the cream, mustard, and parsley; allow the sauce to bubble for a minute or so, stirring constantly. Add salt and pepper to taste.
Pour the sauce over the chicken. (You might like to cover everything with a layer of breadcrumbs, or Parmesan.) Bake in a gas mark 5/190C oven for about 15 minutes, or until bubbling and warmed through.