I want to make consomme. I think I have the general theory down. Im just wondering what veggies works best in the mirepoix, and in what way they should be chopped?
I wonder if the French tradition teaches something specific in this regard?
Do you mean mirepoix? The standard combination is 2 parts onion to 1 part each of carrot and celery. See that linked Wikipedia article for alternatives and the related sofrito and Cajun "holy trinity".