1. Trim beet stems to 1 inch; gently wash, and place in a 3 1/2-qt. saucepan with water to cover. Bring to a boil over medium-high heat. Reduce heat to medium-low, and simmer 25 to 30 minutes or until tender. Remove beets from heat; drain, rinse, and cool completely (about 25 to 30 minutes). Peel and cut beets into eighths.
2. Whisk together orange zest and next 3 ingredients. Gradually add olive oil in a slow, steady stream, whisking until blended.
3. Line a serving platter with watercress. Arrange beets and avocado slices in alternating rows (2 each) on top of watercress. Sprinkle with goat cheese, onion, and freshly ground pepper to taste. Serve with orange vinaigrette.
*1 (6-oz.) package fresh baby spinach may be substituted.