Peel and dice the onion. Peel the carrots and cut into 1/4 inch thick half moons. Cut celery into 1/4 inch thick slices as well. Peel the potatoes and cut into 1/2 inch cubes.
HELPFUL TIP: save all your veggie scraps in a freezer bag, they will come in handy in one of my upcoming posts on making your own veggie stock!