Grilled Halibut with Tatsoi and Spicy Thai Chiles

5 tablespoons sugar
5 tablespoons fish sauce
1/4 cup water
3 tablespoons fresh lime juice
2 tablespoons minced peeled fresh ginger
2 garlic cloves, minced
2 Thai bird chiles with seeds or 1/2 large jalapeño chile with seeds, minced
1 small carrot, peeled, cut into matchstick-size strips
4 6- to 7-ounce halibut fillets
3 tablespoons vegetable oil, divided
1 onion, thinly sliced
3/4 pound tatsoi(about 12 cups packed) 

Mix first 7 ingredients in medium glass bowl.

Season sauce to taste with salt and pepper. (Sauce can be prepared 2 days ahead. Cover and refrigerate.) Prepare barbecue (medium-high heat).

Place carrot in medium bowl. Cover with ice water. Let stand 15 minutes, then drain well. Brush fish on all sides with 2 tablespoons oil. Sprinkle with salt and pepper. Grill until just opaque in center, about 4 minutes per side. Meanwhile, heat 1 tablespoon oil in large nonstick skillet over medium heat. Add onion; stir 1 minute. Add tatsoi; sprinkle with salt. Toss until tatsoi is wilted but still bright green, about 2 minutes; divide among 4 plates.

Place fish atop tatsoi. Sprinkle each fillet with carrot; drizzle each with 2 tablespoons sauce. Serve, passing remaining sauce separately.