Veal stock, methodically reduced 80%, and skimmed and strained over the better part of two days, yields a brown, jellied, concentrated and brilliant-textured sauce known as demi glace.  In the Cathedral of French Culinary Arts, demi glace has its own alcove—less visited than in years past, but still frequented by the faithful who know and appreciate the mystic powers of demi glace as a mother sauce. 

This stuff is expensive to make (Let's say: one cook, two shifts at $15/hour.)  The good news is that a little goes a long way; it keeps for months in the fridge and forever in the freezer.  The breaking news is that now you can buy an acceptable version of veal demi glace at Williams-Sonoma.  The lingering bad news is that it too is expensive (about $23 for 10 ounces).  I've tried it four times now and it compares OK with my own demi glace, which I'll keep making. 

It's worth the cost if, for years, you've longed to try your hand at constructing one of the many classic demi-glace-based French sauces, such as peppercorn, Bordelaise, Roquefort, Robert (mustard), Hunter (tomato/tarragon), orange, various mushroom sauces and others. 

Here we use it to make a classic green peppercorn sauce.

Green peppercorns are early-picked black peppercorns.  They are not solid or hard and therefore soften out in the cooking process.  A green peppercorn from the jar gives way easily as you bite down on it, in contrast to a hard black peppercorn.  They are quite mild, giving off heat only in the aftertaste. 

Beef Tenderloin Tips with Green Peppercorn Sauce

Yield:  Six servings
Preparation time:  30 minutes
Time to serve:  about 5 minutes
See abbreviations, if needed.
·   1.5 lbs        beef tenderloin 
·   some          beef tenderloin scraps
·   ½ stick       butter, clarified
·   3                shallots, diced
·   5 oz            red wine
·   2 oz            cognac
·   2 T             green peppercorns, washed and drained
·   1.5T           demi glace 
·   2T              fresh parsley leaves, sliced, for garnish

For the beef:
1.  Cut the beef tenderloin into 1x1 inch cubes, about 6 cubes per serving
2.  In a very hot heavy skillet, sear the beef tips and beef scraps in butter, to blue 
    (blue = seared nicely on the outside—cool and raw inside)
3.  Remove the beef tips to another heavy skillet and set aside
4.  Continue to sauté the beef scraps to build up more sticky brown mass in the skillet
5.  Remove and discard the scraps
For the sauce:
6.  Over medium-low heat, deglaze the skillet with a little water
7.  Add some butter and sweat the shallots to transparent
8.  Add the wine and reduce
9.  Add drippings from the set-aside beef tips
10.  Off heat, add the cognac
11.  Return to skillet to fire and tip it to ignite the cognac (optional)
12.  Add the peppercorns
13.  Add the demi glace and simmer for 5 minutes before tasting
14.  Adjust seasoning and add a little water if needed for volume 
15.  Set sauce skillet aside to await the beef
For the dish: (a la minute):
16.  Heat plates in a warming oven (with the bread and veggies)
17.  Bring sauce to simmer
18.  Taste sauce and adjust seasoning (use only S/P and water at this point)
19.  Fire the beef skillet to medium high, add a little butter and sauté the 
       beef tips to desired doneness (rare = 120F).  Remove from heat
To plate:
20.  Neatly ladle about 2 ounces of the sauce onto the center of each heated plate
21.  Arrange tenderloin tips over the sauce.  Add garnish
22.  Serve immediately
Notes: 
1.  Of course you can substitute six 6-ounce filet magnons for the beef tenderloin tips.
2.  For rather more elegance, strain the sauce to remove the onions and peppercorns.  Then reheat it in a clean pan with a little clarified butter just before serving.  Be advised, you will lose a lot of volume.  So, if you plan to strain the sauce, also plan to make more of it than called for in this recipe.
3.  Françoise Dionot, founder/chef of L’Academie de Cuisine, insists that pouring sauce over a steak is hugely improper.  "The sauce must be ladled onto the center of the plate and the steak then placed on top of the sauce."  I concur, wholeheartedly.  Besides, one never argued with Françoise.