One of my colleagues went on a fishing trip to Mexico and came back with 20lbs of mahi-mahi for the office. Limes were also in season in the East Bay, so it was an obvious choice to make ceviche. The fish is cooked by the acid in the limes, then some cilantro and onions are added for texture, color, and flavor. Any white fish works well and chopped jalapeno can add some heat. You can also use lemons or add a variety of ingredients, like fresh tomatoes and Tobasco/hot sauce.
Cut fish into bit-size pieces. Cover with lime and lemon juice. Let sit covered in the refrigerator for 20 minutes, then stir, making sure more of the fish gets exposed to the acidic lime and lemon juices. During the marinating process the fish will change from pinkish grey and translucent, to whiter in color and opaque. Once the fish is all white, about 30-45 minutes total from when you covered it with lime juice, stir in the other ingredients. Serve with tortilla chips.