Cut away the peel and pith from the grapefruit and oranges, ensuring all white membrane is removed. Slice each fruit in half pole-to-pole, then slice crosswise into thin pieces. In a bowl, toss the fruit with the sugar and salt and let it sit for 15 minutes to allow the juices to pool.

Melt the butter in a small skillet over medium heat. Add the pecans and salt and cook, stirring frequently, until the nuts are lightly browned and smell fragrant, about 2 to 4 minutes. Transfer the nuts to a paper towel–lined plate to cool.

Drain the fruit through a colander set over a bowl, reserving at least two tablespoons of the released juice. Arrange the fruit slices in an even layer on a large platter and drizzle with the olive oil.

In a medium bowl, whisk the reserved citrus juice with the shallot and mustard. Add the watercress, half of the cranberries, and half of the pecans, tossing gently to coat the greens with the dressing.

Arrange the watercress mixture over the center of the fruit, leaving a border of citrus visible. Sprinkle the remaining cranberries and pecans over the top, season with salt and pepper, and serve immediately.