Ceviche, or seafood "cooked" in lemon or lime juice, is one of the most humble, rustic dishes ever created. And although Peruvian food scholars date ceviche to the 16th century, it's doubtful the cooks that prepared it knew the complex science behind "cooking" protein with acid. When you apply heat to meat, its protein fibers denature, or uncoil from their tightly wound forms and bind in a mesh-like pattern. Acid has the same effect on protein, especially in fish such as tuna. Technically, lemon juice doesn't cook the tuna, but it has the same effect as heat.
Cut the tuna into slices or cubes and place it in a nonreactive bowl. Nonreactive materials include stainless steel, plastic and glass.
Pour lemon juice over the tuna in the bowl and stir to coat it using a plastic, wooden or stainless-steel spoon. A basic ceviche comprises eight parts seafood to one part acid.
Add a few simple, fresh flavoring ingredients to the ceviche. Just about any ingredient goes well with lemon-marinated tuna, but ceviche is best prepared using only three or four of the freshest ingredients. Classic preparations call for chilies, sliced onions and roughly chopped cilantro. You could add jalapenos, diced fruit, shallots, minced garlic or freshly chopped dill, to name a few complementary ingredients.
Add a drizzle of oil to the ceviche, if desired. Since you're not cooking the tuna with heat, use a high-quality, flavored oil, such as avocado oil or extra-virgin olive oil.
Season the ceviche to taste with kosher salt and freshly ground black pepper. In addition to seasoning, salt plays a role in "cooking" the ceviche.
Stir the ceviche and cover the bowl with plastic food wrap. Place the bowl in the fridge.
Let the tuna "cook" for one hour per half pound. Stir the ceviche every 30 minutes. If you want to add diced avocado, stir it in right before you serve the ceviche.