Chop the vegetables depending on the recipe you are using them for:
For large-size chunks, ideal for stock, broth or blended soups, roughly chop the vegetables into 1-inch cubes.
For medium-sized chunks, ideal for soups and stews, cut the vegetables into ½-inch cubes. You can leave the celery and carrots sliced, or cut them half lengthwise and then cut into cubes.
For small dice, ideal for sauces or pilafs, cut the vegetables into ¼-inch cubes.
Heat the olive oil in a medium or large skillet or pot, over medium heat. Add the onions, celery and carrots, and stir to coat with the olive oil. Cook, stirring frequently, until the vegetables have softened and the onions are translucent, about 10 minutes.
Continue with the recipe as desired, or allow the mirepoix to cool completely and freeze flat in freezer-safe zip top bags.