Step 1Place the sliced onions in a bowl and cover with cold water. Let sit for 10 to 15 minutes (or longer). Drain and rinse several times with cold water. Set aside on paper towels.
Step 2Cut the ends off the oranges. Cut away the pith and skin by standing the oranges on one end and cutting down the sides with a sharp knife. Slice the peeled oranges crosswise into thin slices and arrange them on a platter. Arrange the olives and sliced onion over the oranges.
Step 3Whisk together the lemon juice, ground cumin, ½ teaspoon salt and 1/8 teaspoon pepper, or to taste, and olive oil. Pour over the oranges, garnish with the mint and serve.