Mirepoix is a classic French mixture of about 2 parts onion and 1 part each celery and carrot. It is the beginning of many soups, stew and sauces. My version includes leeks and a bit less celery with a bit more carrot. Depending on the recipe you may need a 1/2 C to a cup. To make life easier, I use the processor and do a bunch of it at a time. Cut the veggies into pieces then pulse until chopped. Using the pulse button gives you a more uniform end product. I then freeze the remainder in 1/2 cup portions for recipease©.
When I am making a bunch, I use 3 carrots,well washed (I use organic so I don’t bother peeling. It also saves more nutrients) 2 celery ribs with their leaves, which add a lot of flavor, a leek, white and light green parts and 1 large onion. You don’t need to be exact. If your leek is smaller or your carrot larger, it will not make much difference as long as you have approximately the same amount of each ingredient. I know I said above 2 parts onion, but I am using leeks too which is your missing onion part. You will notice in the last picture, I didn’t bother to chop up the mirepoix, I am just sweating the veggies in the pan (you can see the beads of “sweat” on the veggies) to use in a soup that will be pureed so why should I bother chopping twice.