An essential prepared ingredient in French cooking both now and in antiquity.
At its most simple, mirepoix is chopped carrots, celery and onion, sautéed until softened, but not browned. This recipe from “French Domestic Cookery” adds a bit of parsley, thyme and a bay leaf. Note that modern recipes may specify 2 parts onion to 1 part each carrot and celery. This one does not. It could be because they figured you already knew, it could be because those ratios were not established then, or it could be that they did not care to write it down.