Cut the top and bottom from an orange, so it sits upright on your cutting board. Cut down, following the shape of the orange, to remove the peel and white pith, leaving only the flesh of the orange. To section the oranges: hold the orange in your hand and cut out the segments, leaving the membrane behind. Put the segments in a large serving bowl, and squeeze any juice from the membrane into a medium bowl. Repeat with the remaining orange.

To the medium bowl with the orange juice, add the lemon juice, mustard, salt, and pepper. Whisk to combine. Whisk in the olive oil in a slow, steady stream to make a smooth, somewhat thick dressing.

To the bowl with the orange segments, add the radicchio, radishes, and olives. Drizzle with the dressing, and toss well. Serve.