PICKLE: Bring vinegar, sugar and salt to a simmer in a small pot over medium-high. Cook, stirring until sugar is dissolved, about 1 min. Place rhubarb in a bowl, then pour in vinegar mixture to cover. Let stand at room temperature for 1 hr.
SALAD: Whisk pickling liquid, Dijon and pepper in a small bowl. Drizzle in oil, whisking constantly.
COMBINE greens, radish, snap peas, pea shoots and chives in a large bowl. Drizzle with some dressing, tossing gently to lightly coat. Transfer to a platter, then scatter rhubarb and sunflower seeds overtop.