What is an essential component of mirepoix in cooking?

Mirepoix is a foundational flavor base used in cooking, particularly in French cuisine, and it primarily consists of a combination of onions, carrots, and celery. The inclusion of onions is crucial because they provide a rich sweetness and depth of flavor when cooked slowly. Onions caramelize and contribute to the overall complexity of dishes, making them a key component in stocks, soups, sauces, and various other recipes. While ingredients like garlic, tomatoes, or even potatoes might be used in numerous culinary applications, they do not define the classic structure of mirepoix, where onions are indispensable for building the initial flavor profile.