For dressing, in small bowl, whisk together mustard, honey, mayonnaise, yogurt, and STEVIA IN THE RAW. Season dressing with salt and pepper to taste. Set aside for 20 minutes, or cover and refrigerate up to 24 hours.
Heat grill to medium-high, 400° F for gas grill.
Place pieces of chicken between 2 sheets of wax paper and pound with smooth mallet or side of heavy knife to even 1/2-inch thickness. Season chicken breasts lightly on both sides with salt and pepper.
Place chicken on oiled grill rack and cook over direct heat for 3 minutes. Turn and grill until chicken is firm to touch and white in center, 2-3 minutes. Transfer grilled chicken to plate, tent loosely with foil, and let sit for 5 minutes. Slice chicken across the grain into 1/2-inch strips.
Arrange 1 1/2 cups spinach in a bed on each of 4 dinner plates. Add one-fourth of tomatoes, and 1 onion slice, separated into rings. Top each plate with one-fourth of the chicken and drizzle on 2 tablespoons dressing. Serve immediately.
Source: Stevia in the Raw