This three-citrus dressing melds the flavors of orange, lemon and lime while honey mustard in the background balances the tartness. The dressing pairs well with spinach and as a marinade for chicken.
Cut chicken lengthwise into 1-inch strips and place in resealable plastic bag.
Place garlic on cutting board and sprinkle on salt. Finely chop garlic, then turn blade sideways, press it against garlic and drag down and to the side to smear garlic and salt together. Repeat chopping and smearing until garlic is a chunky paste. Scoop it up with knife blade and transfer mashed garlic to small bowl. Add orange, lime and lemon juices, cumin, stir in honey mustard, and add pepper to taste. Whisk in 1 tablespoon of oil.
Pour 1/2 cup of dressing into bag with chicken and seal, then massage to coat chicken strips. Marinate chicken in refrigerator for 1 to 2 hours. When ready to cook, use paper towels to pat chicken dry. Discard marinade.
Cook chicken in heated grillpan or over outdoor grill on medium-high heat until pieces are white in center at thickest point, about 5 minutes, turning them over every minute to prevent charring. Transfer to plate and let chicken rest for 5 minutes, then cut into 1-inch pieces.
Arrange 2 cups of spinach in each of four wide, shallow salad bowls and top with one-fourth of chicken. Whisk remaining tablespoon of oil into dressing, then drizzle it over salads. Top with mandarin orange sections. Garnish with chopped walnuts as desired.