Cooking vegetables down is a common cooking practice in the
Louisiana kitchen. You don't do this with every dish, only for the
ones you want to have a special vegetable consistency and color.
Examples are gumbo, stew, jambalaya, roasts, etc.
What I've done here is put together some pictures of the process
along with explanations of each step. I did this batch for a pot
roast. I seared the pot roast, removed it from the pot and continued
to cook it in another pot.
2 onions chopped
On a medium fire with a little oil in the pot I added the onions.
I let them slightly wilt and turned the fire up just a little so
most of the onions would turn brown.
One the onions got the browned stage I wanted I added the bell
pepper and celery. I also added about 1/8 of a cup of water; just
enough to almost cover the bottom of the pot, and, turned the fire
down to just below medium.
Here's where you have to watch things a little more closely. You
should keep a little water in the pot at all times stirring every
five minutes or so. Notice below the brown liquid. That is the
result of both browning the onions and searing the roast.
Below, the trinity has cooked a little more. This is probably 45 minutes
into the process.
And finally here is the result 90 minutes later. Notice that it's
about 1/2 the original size.
From here on you would throw in the roast, or add browned chicken
or whatever and continue to cook as you normally would.