Name the vegetable often used as a base in mirepoix.

Mirepoix is a classic culinary preparation used as a flavor base for soups, sauces, and stews, composed of diced vegetables sautéed to release their flavors. The predominant vegetable in a traditional mirepoix is onion. Onions provide a natural sweetness and depth of flavor as they caramelize during the cooking process, which is essential for many dishes.

While celery and carrots are also integral parts of mirepoix, they serve as complementary ingredients rather than the primary base. Garlic, while flavorful, is not typically included in a standard mirepoix; it is often added separately for additional flavor in many recipes. Therefore, onions are the key component that grounds the mirepoix, making them the essential vegetable that forms the foundation of this flavor base.