My sister's boyfriend made awesome shrimp ceviche for our Thanksgiving dinner. The raw shrimp turned pink as they "cooked" in the lime juice. The juice also cooks the onions and the other ingredients. He mentioned that you can also make ceviche using store-bought, canned tuna. The tuna's already cooked, but the taste is the same. I'd make this with raw fish for a dinner party, but for everyday this tastes great and is ridiculously easy.