Combine tarragon, Greek yogurt, harissa sauce, garlic, vinegar, cumin, and salt in a blender. Puree until smooth, scraping down sides as needed. Pour marinade over chicken breasts in a shallow dish, thoroughly coating both sides. Cover and refrigerate for 2 to 3 hours.

Preheat an outdoor grill for medium-high heat and lightly oil the grate.

Remove chicken breasts from the marinade and place on the pre-heated grill. Cook over medium-high heat until dark grill marks appear, about 7 minutes per side.

Garnish with harissa sauce and a sprig of tarragon.