If you don't wish to use a frozen blend of cut celery, onions and carrots you can always just use fresh cut by washing and chopping your own. This will add an extra minute or two to the cooking process but the textures maybe even better. I like the convenience of the frozen bags though. Use about 1 small onion (1 cup, and 2 celery stalks, and 2 medium carrots if you won't be using the bag of the frozen blend.