Combine vinegar and mustard in a medium bowl. Slowly whisk in olive oil until emulsified.
Toss watercress with some of the dressing and arrange on a platter. Slice heirloom tomatoes, sprinkle with salt and pepper, and add to the platter with the watercress. Top with sea beans and garnish with red pepper brunoise. Drizzle with additional dressing, if desired.
Yield: 6 servings