To serve, whisk the remaining 2 tablespoons vinegar, 2 teaspoons syrup, 1/4 teaspoon salt, oil and pepper in a large bowl. Add watercress and sliced fennel; toss to coat. Arrange the beets around the edge of a platter and drizzle with 2 tablespoons of the pickling liquid. Mound the salad in the center. Top with pistachios and cheese. Garnish with chopped fennel fronds, if desired.