Extracting the flavour of single vegetables for the purpose of sauce I am interested in making vegetable sauces that are concentrates of a particular vegetable. So a concentrated sauce that has just the flavour of a chosen vegetable.

Which vegetables would be suitable to make such concentrated sauces?
What methods are possible to make a refined vegetable sauce?

EDIT: I want to attempt this with either: Carrot, Broccoli, Celery

I will steam the vegetable till softened but not mushy, then puree. I want the result to be a thick puree that I can place a 1/4 teaspoon (or the less the better) on a piece of Sashimi. Should I reduce the puree over low heat?

I am afraid that the result will be too flavourless, Is there a better method to reach the result? Can you detail any mistakes to avoid in the process? (i am not an experienced cook, it is a hobby i am getting into)
Any vegetable can work.  Cook vegetable (let's say carrot, for example), puree, pass through tami...you have a sauce.  As @Stephie suggests above, better advice will depend on your plan.  Vegetables can be juiced...that juice can be clarified (more like the result of the distillation process mentioned by @Fabby).