1. To make Vinaigrette: Toast coriander in dry skillet 1 minute, or until fragrant. Transfer to bowl, and cool completely.
2. Blend toasted coriander seeds, vinegar, cilantro, and shallot in blender or mini-food processor until smooth. With motor still running, add oil, and blend until combined. Season with salt and pepper, if desired.
3. To make Salad: Slice ends off oranges, and stand upright on counter. Cut away peel and pith with serrated knife, exposing fruit. Working over serving bowl to catch juices, release each orange section by slicing between membranes with knife. Remove and discard seeds. Add orange segments and juices to bowl with watercress and jicama. Toss with 1/2 cup Vinaigrette. (Reserve remaining Vinaigrette for another use.) Divide Salad among serving plates, and top with avocado slices (if using).