You want to bring out the natural sweetness of the veggies first before adding in any liquid. Bring a heavy soup pot (I love enameld cast iron like this one) over high heat. Add in 1 tsp oil, if using, or a spray. Next, add in the onion, and a pinch of salt. Sauté for about a minute, then lower the heat. Let it change to a nice golden color just a little bit, but stir it often so it doesn’t burn. Add in the carrots, celery, and garlic, and saute for another 3-5 minutes until they show a little color change and get very aromatic.