Olive-Brine-Marinated Chicken With Date Relish

Name: Olive-Brine-Marinated Chicken With Date Relish
Cuisine: Middle Eastern
Ingredients: Zest and juice of 1 lemon; 2 garlic cloves, finely grated; ⅓ cup olive brine; ¼ tsp. crushed red pepper flakes; 6 sprigs tarragon, leaves picked, chopped, divided; 1 tsp. Diamond Crystal or ½ tsp. Morton kosher salt, plus more; 4 skin-on, bone-in chicken thighs (about 2 lb.); 1 Tbsp. extra-virgin olive oil; 1 small red onion, sliced; 1½ lb. medium carrots, scrubbed, cut on a diagonal into 3" pieces; 2 tsp. sumac; Freshly ground black pepper; 1 small red onion, thinly sliced; Zest and juice of 2 lemons; 6 firm Medjool dates, pitted, coarsely chopped; ⅓ cup coarsely chopped pistachios; ¼ cup coarsely chopped tender herbs (such as tarragon, parsley, and/or chives); ¼ cup green olives, pitted, coarsely chopped; Kosher salt