Clean dirt off mushrooms with a damp rag or paper towel. Cut large mushrooms into quarters and smaller mushrooms in half.

Peel onions and slice into thin rings. Place the mushrooms and onions in a dish with a lid (such as a plastic or glass dish that you use for leftovers).

In a medium bowl, whisk together the Italian dressing mix, olive oil, vinegar, water, and salt.

Pour over the mushrooms and onions, cover the dish with a lid, and shake gently to coat. Place in the refrigerator, and allow to marinate for at least 8 hours.