Marinating fish in citrus adds vibrant flavor, and the juice's acid turns the flesh opaque and firms up its texture, so it looks cooked. Combined with bright aromatics, this dish is cool, light, and simple to make.
Pour lime juice and orange juice through a sieve set over an 8-inch square glass baking dish. Arrange red snapper fillet in a single layer in dish (fish should be completely covered by juice). Cover with plastic wrap and refrigerate until fish is opaque throughout, about 4 hours. Drain fish well; discard marinade.
Divide fish among four plates. Drizzle fish sauce and toasted sesame oil over each portion. Top with chile, scallion, and black sesame seeds. Serve immediately.
Fresh fish is key to ceviche; ask for the best at your fish market.