Step 1

Prepare grill for medium-high heat. Mix onion, cilantro, lime juice, ginger, sugar, chopped mint, half of chiles, and ⅓ cup oil in a medium bowl; season with salt. Set relish aside.

Step 2

Season inside of fish with salt. Stuff with mint sprigs, lime slices, and remaining chiles; tie with kitchen twine. Drizzle with remaining 1 Tbsp. oil; season with salt. Grill fish until skin is crisp and flesh is flaky and opaque down to the bone, about 5 minutes per side. (If a small knife slides easily through the thickest part of flesh, the fish is done.) Serve with relish.

Step 3

DO AHEAD: Fish can be stuffed and tied and relish can be made 6 hours ahead. Cover separately and chill.