Heat oil or butter in a wide pan over medium until shimmering/foamy.
Add onion, carrot, and celery with the salt. Stir to coat and spread into an even layer.
Sweat, don’t brown: cook, stirring occasionally, until vegetables are softened and onion is translucent, about 8–10 minutes for a ¼″ dice. Reduce heat if edges begin to color.
Use immediately as a base for soups, stews, sauces, or cool completely for storage.
Notes
• Ratio: Classic mirepoix is 2:1:1 (onion:carrot:celery) by volume. Prefer a mellower base? A house ratio around 4:3:3 (≈1⅓:1:1) also works — keep dice even. • Dice size: Smaller dice soften faster; larger dice (½″–1″) take longer (see table above). • Storage: Refrigerate airtight up to 4 days. For meal prep, portion into 1-cup freezer bags, flatten, and label; best quality 2–3 months (usable up to 4–6 months). • Dietary: Naturally vegan, vegetarian, and gluten-free.