This salad is the one Debi makes at many of their family gatherings in Tuscany. Avocados are still a novelty in Italy, and many Italians don't usually use them as an ingredient. It's easy to make and is always a crowd pleaser.
Lay out the radicchio leaves in a large bowl or platter. Add the tomatoes, avocados and onions. Lightly dress the salad with the lemon juice and oil. Toss to combine. Sprinkle with salt and pepper. Serve immediately.