Using a sharp knife, cut and peel the oranges removing the white pith. Peel the mandarins.
Thinly slice the oranges crosswise, removing any pits.
Arrange the oranges on a platter and scatter the fennel, red onion and Sicilian olives on top.
Drizzle with the olive oil, lemon and season with Maldon salt and freshly ground cracked black pepper.
Garnish with the basil and serve.