Combine green curry with soy sauce, slowly stir in coconut milk. Add salt, pepper, and granulated onion.
Place halibut in a large enough glass dish. Pour mixture over top. Using tongs, gently turn to coat. Cover dish with plastic wrap and place into refrigerator for 2 hours.
In a small saucepan, heat oil. Add ginger and garlic and cook for 15- 30 seconds. Add remaining basting sauce ingredients and bring to a high simmer (light boil). Reduce heat to low and let mixture simmer for 3-4 minutes, stirring occasionally. Sauce will be fairly runny and that is okay. Reserve about 2-3 tablespoons of mixture for later.
Preheat grill for medium-high heat. Once up to temperature, oil the grill grates well. Make about 4 passes on the grill to create a non-stick surface for the fish.
Remove fish from marinade place onto the grill. Cook for 5 minutes, gently turn, baste liberally with basting sauce, and let cook another 3-4 minutes. Turn once more and baste liberally again.
Once internal temperature of fish reaches 145 degrees, it is done. Very gently remove fish from grill, place onto a serving dish and top with warmed reserved basting sauce and sliced green onions. Serve immediately.