Smart cooks know that, when there's a heat wave (that would be now, in the Northeast), we ditch the sanctimonious slog of recipes that call for chopping, stirring, stuffing and run like hell for air conditioning and a Tom Collins. Luckily, both go well with ceviche, one of the easiest cook-free meals of all time.
A Little Backstory
Ceviche is traditionally a South American dish made by marinating pieces of fish or shellfish in citrus juice. Essentially, the acid from the juice denatures the proteins just as heat might, giving the seafood a cooked texture and taste—but without any grilling, sautéing or other too-stifling-for-summer activity required.
There are no set rules about how long ceviche should marinate before being served. I think anything under 10 to 30 minutes is too short (although I know some chefs serve theirs moments after the acid touches the fish, giving it a more raw texture and taste), and anything longer than five or six hours (though I guess it's possible, though not ideal, to leave a plate overnight) is probably too long. After all, freshness is mostly the point.
How to Make It
1. Prep the raw fish by trimming it into thin slices or bite-size pieces. (To extract shellfish like lobster, you may find it easier to quickly blanch the whole lobster first. Some cooks will blanch and shock all of their fish for 30 seconds before adding to the dish, but I think par-cooking feels like cheating, and I find the fish takes on a chewier texture.) Since the acid will be cooking the food on contact, smaller pieces will allow the change to happen more quickly. Most fish will work here, but scallops have an especially fresh taste and smooth, slippery texture.
2. Choose your aromatics. Since the acid and salt will take effect on them, too, consider adding ingredients to the dish in the order you might like them to cook—flavors that could afford a little mellowing, such as onions or chili peppers, may do well to hit the citrus pool before, say, leafy herbs that might wilt and discolor. You can garnish with delicate items like these just before serving.
3. Add the citrus. Lemon and lime are the top two choices, since orange and grapefruit tend to be a little less acidic, but combinations can be fun and extra flavorful. You don't really need any fat here, though I prefer to add a splash of canola oil, and sometimes a dash of hot sauce, to give the liquid some body. Add salt and any seasonings. I like to make a dressing the way you might a vinaigrette—mix it in separate bowl, taste for seasoning and balance, and apply. How soupy or dry you like it is up to you.
4. Let the dish marinate in the fridge for your desired time, and chill a serving plate to use for later. Watch (and taste) as you go—the flesh will go from gray and translucent to firm, whitish-pink, and opaque. Remove from the fridge and serve with garnishes and something crunchy for texture.