Slice the top and bottom off the grapefruits and oranges. Stand the fruit upright on a cutting board and cut away the peel and pith, following the curve of the fruit. Working over a bowl to catch the juice, cut between the membranes to release the segments.

Toss the citrus segments with the sugar and salt. Let them stand in a colander set over a bowl for 15 minutes — this draws out excess moisture so the salad doesn't become watery. Reserve 2 tablespoons of the collected juice for the dressing.

Melt the butter in a small skillet over medium heat. Add the pecans and a pinch of salt and cook, stirring frequently, until the nuts are fragrant and deeply toasted, about 3 to 5 minutes. Transfer to a small plate to cool.

Whisk the olive oil, reserved citrus juice, shallot, and mustard together in a large bowl. Season the dressing with salt and pepper to taste.

Add the watercress and cranberries to the bowl with the dressing and toss gently until the greens are evenly coated.

Arrange the dressed greens on a large platter or individual plates. Top with the drained citrus segments and toasted pecans and serve immediately.