Wash and dry the shoots and mint, place in a large bowl.
In a mason jar, combine the mustard, water, lemon juice, oil, creme fraiche, sugar, salt, and pepper. Put the cap on and shake vigorously to emulsify. Drizzle the vinaigrette over the greens and toss well to coat. Serve immediately. Store any remaining vinaigrette in the jar in the refrigerator.