Stir together salt, coriander and cayenne in a small bowl.
Peel oranges and remove as much bitter white pith as possible with a small, sharp knife. Slice crosswise into 1/2-inch thick slices. Arrange on a large serving platter and sprinkle with spice mixture. (Salad may be prepared to this point and stored, covered, at cool room temperature for up to 3 hours.) Top with red onions and serve.