In a large skillet, cook the pancetta over moderate heat until browned and crisp. Drain on paper towels and let cool, then coarsely crumble.
Peel the oranges with a sharp knife, removing all of the bitter white pith. Working over a strainer set over a bowl, cut in between the membranes to release the sections. Squeeze the membranes to extract the juice: You should have 3 tablespoons. Pour the juice into a jar. Add the olive and sesame oils, season with salt and pepper and shake. Halve the orange sections.
In a large bowl, toss the papaya, scallions, watercress and oranges with the dressing. Season the salad with salt and pepper and transfer to plates. Garnish with the crumbled pancetta and serve.