SLICE: Zest 2 of the oranges to get the 2 tablespoons (10 g) of orange zest for the tapenade. Set aside for later. Use a serrated knife to remove the rest of the skin and pith. Slice each orange into 5–6 slices and set aside.
BLITZ: Add the orange zest, olives, garlic, lemon juice, parsley and rosemary to the bowl of a food processor. Pulse until everything begins to combine but is still chunky. Scrape down the sides of the bowl and slowly add the olive oil until the tapenade is combined.
ASSEMBLE: Layer the orange slices onto plates and top with the red onion slices. Spoon over the olive rosemary tapenade. Drizzle with a little more extra-virgin olive oil, if you wish.