Step 1In 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 2To 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.