2Combine first 6 ingredients in medium skillet. Add 1 to 2 teaspoons serrano chile, depending on level of heat desired; boil until sauce thickens slightly and is reduced to generous 3/4 cup, 8 to 9 minutes. Remove from heat; stir in 2 tablespoons cilantro and 2 tablespoons green onions. Season sauce with salt and pepper.
3Brush fish all over with 1/4 cup sauce; sprinkle with salt and pepper.
4Grill fish until opaque in center and grill marks appear, 5 to 7 minutes per side, depending on thickness of fish.
5Divide coconut sauce among 4 plates; top with fish. Sprinkle with remaining cilantro and green onions.