Salt and pepper to taste
Combine the dressing ingredients in food processor and blend until smooth. Taste and adjust to a tart-sweet balance by adding more sugar or vinegar. Refrigerate until about an hour before dressing salad. Dressing should be at room temperature.
Trim beans of stem ends and steam or boil until tender with a little firmness. Immediately drain in a colander and cool down with cold running water. Can be refrigerated overnight.
Place the sliced onion in a bowl and sprinkle generously with vinegar. Soak about 30 minutes to rid the onion of its heat. Drain before using.
To assemble the salad: Discard any bruised salad greens. On a large platter, arrange the greens in several clusters. Scatter half of the Napa cabbage over the bottom of a plate around the clusters. Top with half the beans, a scattering of half the orange segments and onion rings, and all the mint leaves.