Whisk together 1/2 cup (125 mL) barbecue sauce and maple syrup in a large shallow bowl or glass baking dish. Add steelhead fillet and turn to coat.
Mix carrots, balsamic vinegar and olive oil in a medium bowl. Season with salt and pepper, and set aside.
Preheat grill to medium-high. Set soaked planks on hot grill, lightly brush them with vegetable oil, and close the grill. When grill starts to smoke, remove fillet from marinade, season with salt and pepper to taste, and arrange fillets on plank. Brush some additional marinade over salmon, close the lid, and reduce heat to medium. Cook without peeking for 12 min.
Brush additional 2 tbsp (30 mL) barbecue sauce over fillet and cook for 5 to 10 min. longer, until opaque and just cooked through. Meanwhile, mix chives into carrot slaw. Place slaw on a platter and top with maple steelhead fillet to serve.