Whisk together mustard, orange juice, orange zest, olive oil, tarragon, sugar, salt, and pepper in small bowl;

set aside.

Whisk together orange juice, remaining 3 tablespoons olive oil, remaining teaspoon sugar, 1 1/2 teaspoons salt, 1/2 teaspoon pepper, garlic, and water in medium bowl.

Place marinade and chicken in gallon-size zipper-lock bag and toss to coat;

press out as much air as possible and seal bag.

Refrigerate 30 minutes, flipping bag after 15 minutes.

Turn all burners to high and heat with lid down until very hot, about 15 minutes.

Scrape grate clean with grill brush.

Dip wad of paper towels in oil; holding wad with tongs, wipe cooking grate.

Leave primary burner on high and turn off other burner(s).

Remove chicken from bag, allowing excess marinade to drip off.

Place chicken on cooler side of grill, smooth-side down, with thicker side facing lit burner.

Cover and cook until bottom of chicken just begins to develop light grill marks and is no longer translucent, 6 to 9 minutes.

Using tongs, flip chicken and rotate so that thinner side faces lit burner.

Cover and continue to cook until chicken is opaque and firm to touch and instant-read thermometer inserted into thickest part of chicken registers 140 degrees, 6 to 9 minutes longer.

Move chicken to hotter side of grill and cook, uncovered, until dark grill marks appear, 2 to 3 minutes.

Using tongs, flip chicken and cook until dark grill marks appear and instant-read thermometer inserted into thickest part of chicken registers 160 degrees, 2 to 3 minutes longer.

Transfer chicken to a cutting board, let rest, tented with foil, 5 minutes.

Slice each breast on bias into 1/4-inch-thick slices and transfer to individual plates.

Drizzle with reserved dressing and serve.