Quarter and cut the radicchio finely into thin ribbons. Cut the onion in half and thinly slice into half moons. Add to bowl and mix well, making sure onions are separated and mixed throughout the lettuce.
Drizzle oil and vinegar. Add salt and parmesan. Mix well with 2 forks.
Finish with a dusting of parmesan.
This salad is made to order, it doesn’t keep well. So I prep a head of radicchio and keep in the fridge for up to a week. It makes about 4 cups. Then when I want a salad I take it out and make about a cup at a time.