Ceviche is a popular seafood dish made by combining raw seafood in citrus juice. The acid in the juice changes the structure of the protein and “cooks” the meat, making it turn opaque.
By using “sushi grade” tuna (a premium cut that has been properly stored and adequately frozen to remove the threat of parasites), you can enjoy this dish more safely.
For this recipe, I like to sear my tuna on the grill first, giving it that hint of a smoky flavor. This isn’t a traditional way to do ceviche, but it really tastes delicious. Only sear it briefly though; you want it to be very rare when you take it off the grill so the lime juice can work its magic.
For more details on the science behind it, read this.