Heat the butter and olive oil in a large pot over medium heat until the butter is melted and lightly foamy. Add the carrots, celery, and onion along with a pinch of salt, and cook for 5–7 minutes, stirring occasionally, until the vegetables are softened and fragrant but not browned. This step builds the base flavor of the soup, so don’t rush it.
2 medium carrots, ½ onion, 1 tablespoon butter, 1 tablespoon extra virgin olive oil, 2 celery ribs