1. Combine vinegar, oil, green onions, parsley, salt, dry mustard, basil and pepper, stir well.
2. Set aside.
3. Carefully remove and discard stems (or freeze for later use) Add caps to vinegar mixture, toss gently to coat evenly.
4. Fridge for 4 hours.
5. Keep up airtight container.
6. *When I make this for home I ususally use the stems but you don't have to, otherwise I will freeze the stems for other recipes such as sauces.