The sauce: in a bowl, mix all the sauce ingredients together; mix until completely blended and smooth; taste for seasoning.
Place halibut in a zip-lock plastic bag and pour ¼ cup of the sauce over it; turn the fish in the bag to coat evenly; seal bag and marinate fish for at least 30 minutes and up to 2 hours in the refrigerator.
Prepare grill for med-high heat grilling or a grill pan over high heat.
Spray grill pan with cooking spray; remove fillets from the marinade and place on the grill or in grill pan.
Cook for about 5 minutes per side, or until cooked through and opaque.
Place the fillets on serving plates and spoon over some of the remaining sauce; garnish with the scallions and serve immediately.