Cut off the top and bottom ends of the oranges. Stand an orange on one end and slice off the rind where it meets the flesh. Repeat with the second orange. Run a knife along the membranes that separate each section, pull off the orange segments, and set aside in a small bowl. Squeeze the membranes over a small bowl to extract the orange juice. Set aside 1 tablespoon of the juice and discard the rest.

Cut the fennel in half lengthwise and cut out the hard core. Slice the fennel crosswise into ¼-inch-thick slices. Heat the olive oil in a large skillet over high heat. Add the fennel slices and cook, without stirring, until the undersides are lightly browned, about 2 minutes. Stir in the lemon juice and cook until the fennel is crisp-tender, about 1 minute. Season to taste with salt and pepper. Transfer to a dish and cool.

To make the vinaigrette, whisk together the lemon and orange juices in a medium bowl. Slowly whisk in the olive oil. Season to taste with salt and pepper.

Combine the fennel, orange segments, and 2 tablespoons of the vinaigrette in a medium bowl. Toss gently and season with salt and pepper. Toss the watercress with the remaining vinaigrette and season to taste with salt and pepper. Place equal amounts of the watercress on 4 plates and top with the fennel-orange mixture. Serve immediately.