Dry the brill with kitchen paper. Chop the thyme leaves finely, discarding the stems. Combine the thyme with the garlic, lemon zest, olive oil and seasoning in a dish big enough to take the brill. Add the brill to the marinade and with your hands coat it all over. Cover and leave to marinate for at least an hour in the fridge. When ready to cook, heat the grill to high. Place the fish on a grill rack and cook for 4 minutes on each side.