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. In a large bowl, combine avocado, tomatoes, jalapeno, cilantro, and olive oil. Season with salt and pepper. Serve fish topped with avocado salad and lime wedges alongside.
Fresh fish is key to ceviche; ask for the best at your fish market.