Marinate the Chicken. In a large bowl, mix the lemon juice, minced garlic, olive oil, salt, and pepper. Add the chicken thighs and rub the marinade into the skin and flesh. Cover and refrigerate for at least an hour, or up to overnight.
Make the Glaze. In a small saucepan over low heat, combine the apricot jam, Dijon mustard, cider vinegar, grated ginger, lemon juice, and cayenne (if using). Stir until smooth and warmed through, then remove from heat. Set aside.
Preheat the Grill. Heat your BBQ to a medium-high heat. You want a steady heat without big flare-ups. If using charcoal, let the coals turn grey before cooking. Oil the grill grates lightly to prevent sticking.
Start Grilling. Place the chicken thighs skin-side down over direct heat. Grill for 5–6 minutes until the skin starts to brown and crisp. Flip the thighs and move them to indirect heat. Cover the grill and cook for another 15–20 minutes.
Baste with the Glaze. Start brushing on the glaze once the thighs are nearly cooked through. Brush a thin layer over each thigh, then close the lid and cook for 3–4 minutes. Repeat this process two or three times to build up a sticky coating.
Finish and Rest. Once the internal temperature of the chicken reaches 75°C, move the thighs off the heat and give them one final brush with the glaze. Let them rest for 5 minutes before serving.