Cut the fennel in half and then quarters. Using a mandoline (use the hand grip for protection against the blades!), carefully slice the fennel into thin slices; reserve the fennel fronds to add in later. Trim the skin off the oranges and cut out the fruit segments and place them in a bowl.
Layer the slices of fennel and the segmented oranges on to a flat large serving platter and drizzle heavily with olive oil. Season with sea salt and cracked pepper to taste.
Chop some of the fennel fronds and scatter over the top generously. If you like, you can zest some of the orange skin to add to the salad right before serving.