Thaw fish, if frozen. Rinse fish; pat dry with paper towels. In a small bowl stir together scallions, 2 teaspoons of the lemon juice, the ginger, and garlic.
Arrange fish fillets in a single layer in a microwave-safe shallow baking dish, tucking under any thin edges. Spoon scallion mixture over fish. Cover dish with vented plastic wrap. Microwave on 100-percent power (high) for 3 to 5 minutes or until fish flakes easily when tested with a fork, giving the dish a half-turn halfway through cooking.
Using a slotted spatula, transfer fish to dinner plates. Stir together fish sauce, jalapeno pepper, and remaining 2 teaspoons lemon juice; drizzle over fish. Makes 4 servings.