Cut fish into half-inch cubes. Peel, deseed, finely dice and then mince jalapeno pepper. Finely dice sweet white onion and mince garlic clove. Pick cilantro.
Zest and juice limes and lemon, reserving zest. Combine above ingredients with olive oil in a bowl and refrigerate for an hour. Zest, then remove grapefruit peel and cut segments from the pith. Peel and seed avocados and cut into small cubes. Combine lime, lemon and grapefruit zest and rough chop. Stir together marinated seafood mixture with grapefruit segments, avocado cubes and zests. Salt and pepper to taste. Serve with white corn tortilla chips.
About ceviche: The underlying idea of ceviche is that it is a cold soup made by "cooking" seafood in an acidic base (usually lemon or lime juice) as opposed to a heat source. Traditionally it is served as hors d'oeuvre. Technically speaking, the chemical process that occurs between the citrus and the fish does not actually cook the fish. The acid that penetrates the fish prevents the growth of microorganisms and softens the flesh fibers at the same time. The acidified flesh takes on an opaque appearance and firm texture, but the fish is, for all intents and purposes, closer to raw than cooked. As such, some seafood poses greater safety risks than others when prepared and eaten in this way, especially for people with compromised immune systems or expectant mothers. Ask your fishmonger for safety recommendations and guidelines when selecting fish for your ceviche. Each country in Latin America has its own version of ceviche, incorporating some or all of the following ingredients depending on region and availability: Very fresh-caught fish (tuna, squid, swordfish, flounder, red snapper or other white fish) Citrus juice (lime, lemon, sour orange) Tomatoes, chili peppers, cilantro, onion, garlic, salt and pepper Shellfish: shrimp and/or lobster, scallops, oysters are another variation, but they should be partially cooked before including for food safety. Ceviche is considered the national dish of Peru, where it is served with slices of cold sweet potatoes or corn-on-the-cob.