In a small bowl, whisk together the Dijon mustard, orange juice, orange zest, olive oil, tarragon, sugar, salt, and pepper. Set aside for serving later.
Add the chicken and marinade to a gallon-size zipper-lock bag. Seal tightly, pressing out as much air as possible, and turn to coat the chicken evenly.
3-4 Chicken Breasts
Refrigerate for 30 minutes, flipping the bag once halfway through.
To Grill
Preheat the grill by turning all burners to high and closing the lid for about 15 minutes, until very hot.
Scrape the grates clean with a grill brush.
Using tongs, dip a wad of paper towels in oil and rub the grates to prevent sticking.
Vegetable Oil
Turn off all burners except one. Leave the primary burner on high.
Remove the chicken from the marinade and let the excess drip off. Place the chicken smooth-side down on the cooler side of the grill, with the thicker end facing the heat.
Cover and cook for 6-9 minutes, until the bottom develops light grill marks and is no longer translucent.
Flip the chicken and rotate so the thinner side faces the heat. Cover again and cook another 6-9 minutes, until the chicken is firm and a thermometer reads 140°F in the thickest part.
Move the chicken over to the hot side of the grill. Cook uncovered for 2-3 minutes per side, until dark grill marks appear and the internal temperature reaches 160°F.
Transfer to a cutting board, tent with foil, and let rest for 5 minutes.
Slice each breast on the bias into ¼-inch slices. Serve warm, drizzled with the reserved dressing.
Drizzle with reserved dressing and serve.
Notes
Marinated raw chicken can be frozen for up to 3 months - just thaw overnight before grilling.Leftover grilled chicken will keep in an airtight container in the fridge for 3-4 days.