Begin by slicing the top and bottom of each orange to reveal the flesh. Set a cut side down on the cutting board and use your knife to slice down the sides to remove the peel and as much white pith as possible. Turn the oranges on their sides and slice into rounds.

Use a sharp knife or a mandoline to shave the fennel and red onion into very thin slices.

Arrange the oranges, fennel and onion on a platter, layering the fennel and onion between the slices of orange.

Drizzle with olive oil, then sprinkle with pistachios and sea salt. Scatter the reserved fennel fronds around the plate if desired. Serve immediately.