Step 1 In a small bowl, whisk together the mustard, vinegar, oil, a heaping 1/2 teaspoon salt and several grinds of pepper. Place the beet greens in a large bowl and pour over 1 to 2 tablespoons of the vinaigrette. Massage the vinaigrette into the greens. As the greens soften, they will shrink in volume and darken in color. Set aside.
Step 2 To the remaining vinaigrette, whisk in the creme fraiche, chives and tarragon. This makes about 1 cup dressing, more than is needed for the remainder of the recipe; the dressing will keep, covered and refrigerated, up to 3 days.
Step 3Toss the greens with the shallots, along with 3 more tablespoons of the dressing, or enough to lightly coat the greens. Arrange the salad on a serving platter and garnish with the toasted walnuts. Serve immediately.