Mustard Caper Sauce

This sauce may be my favorite.  It is delicious on chicken or pork.  It is a classic on sweetbreads.
It is so rich the word sinful comes to mind.  It is best made with the very rich and very salty
broth saved from a roast chicken.

Ingredients:
Half an onion (about 4 ounces or 75g), finely diced
Half a cup (125 ml) of chicken stock  You may actually use less.
If you have it, the broth from a roast chicken
Half a cup (125 ml) heavy cream
A tablespoon (15 ml) of prepared Dijon mustard
A tablespoon (15 ml) of capers, rinsed
A tablespoon or more of roux

Method:
This sauce can be made with or without the saved broth from a roast chicken.  It is better with it;
it will be very good without it.  The saved broth is very salty so if you don't have it you must add
enough salt to your stock to make it very salty.  The cream added at the end corrects the excess
saltiness.  The onion adds flavor but mostly adds texture and body to the sauce.  The onion should
be cooked enough to take away the raw taste and the crunchiness.  The simplest way to do this is
to dice it fine then microwave it until it is cooked thoroughly.
If you have the saved broth, add chicken stock to make up a volume of half a cup (125ml).  If you
don't have it, measure out a half cup (125 ml) of chicken stock and add enough salt to make it very
salty.  Warm this in a small saucepan and stir in the roux.  While it is warming microwave (or
sautée gently without browning)) the diced onion.  When the onion has lost its crunchiness add it
to the thickened stock.
At this point the sauce should be too salty and too thick.  If it isn't, correct it.  When it's right stir
in the mustard and the capers.  You may want more of each or both.  The sauce should be over-seasoned.
Now you fix everything.  Add the heavy cream and stir.  You are done.
The sauce should be flavorful, tangy, properly salty, and luxurious!  Enjoy it!