1. Peel the citrus, removing as much as the pith as possible, and slice into wheels, removing any seeds. (To peel citrus: Slice off both ends and stand the fruit upright on the cutting board; using a sharp knife, cut the rind downward, following the natural curve of the fruit. Continue around until all peel and pith are removed.)

2. Arrange the citrus wheels and avocado slices on a serving platter and scatter the shallot and radish over the top. Drizzle with lime juice and season with salt. Garnish with cilantro if desired. If making ahead, refrigerate prepared salad without avocado; slice and add the avocado when ready to serve.