Use a sharp knife to cut the skin and white pith off all the oranges, then slice into thin, even slices. Alternatively, you can slice first and remove skin second.

Place all the orange slices in a large mixing bowl. Add the thin slices of fennel and red onion and drizzle with the vinaigrette. Gently toss to combine. Add the avocado last, to prevent getting squished during the mixing or add to each plate individually upon serving along with the fennel leaves for garnish.