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".
If available, parsley root and parsnip work very well in addition to the classic mix.
Also roasting a cut onion in a pan until black and adding it to the stock with the peel gives additional depth and colour