How to Make It
Finely grate 1 teaspoon of zest from the oranges and reserve. Using a sharp knife, peel the oranges, taking care to remove all of the bitter white pith. Working over a bowl, cut in between the membranes to release the orange sections into the bowl. Pour off the juice, reserving 2 tablespoons for the vinaigrette.
In a blender, combine the vinegar with the reserved orange zest and juice, the garlic, shallot, dill, mustard and ground fennel seeds. With the machine on, slowly pour in 1/2 cup plus 2 tablespoons of the olive oil; season with salt and pepper.
In a large skillet, toast the walnuts over moderate heat until fragrant, about 3 minutes. Transfer to a plate to cool. In the same skillet, heat the remaining 2 tablespoons of olive oil. Season the shrimp with salt and pepper and cook, turning once, until opaque throughout, about 4 minutes.
In a large bowl, toss the watercress with the fennel, olives, toasted walnuts and orange sections. Add 1/2 cup of the vinaigrette and toss well. Transfer the salad to plates, top with the shrimp and serve, passing the remaining vinaigrette at the table.