Heat a grill pan over medium-high heat and coat with olive oil cooking spray.
Season the chicken well with sea salt, freshly cracked pepper, and garlic powder to taste.
Place the chicken on the grill pan and cook for 6-7 minutes. Flip over and grill for another 6-7 minutes, or until a meat thermometer reads 155-160 degrees. Set aside to rest with a foil tent.
Meanwhile, prepare the sauce by whisking the Dijon, vinegar, water, olive oil, tarragon, sugar, sea salt, and freshly cracked pepper to taste in a small saucepan over medium heat for 5-7 minutes, or until warmed.
Slice the chicken, drizzle sauce over it, and serve the remaining sauce. Enjoy.