What if you purchased too many onions, bell peppers, and celery, or your garden has supplied an abundance and you fear they'll go bad before you can use them? What can you do before it all goes to waste? It's simple: you can freeze them.
Peel the outer layers of the onion away so you can dice it. Wash the bell pepper and celery. (see notes)
Dice the onions, bell peppers, and celery.
Place the diced Trinity on a parchment paper-lined cookie sheet in as much of a single layer as possible. Carefully place the cookie sheet in the freezer for about 6-8 hours. See notes.
Once the diced Trinity is frozen, either add all of the onions, bell peppers, and celery to one large freezer bag ( this can be helpful if you know that you are going to use the Trinity again) or place each one in different freezer bags (this can be helpful if you just plan on grabbing what you need. Freezing the vegetables in a single layer on the cookie sheet before bagging them makes it easier to scoop out little bits as you need them).
Notes
After washing, set the bell pepper and celery aside so that they can air dry, or use a paper towel and give them a dry. This can help prevent some ice crystals from forming on the celery.
Freezing the onions, bell peppers, and celery in a single layer on the cookie sheet will help ensure that the pieces are individually frozen and not stuck together in a giant clump.