In a large bowl, combine the tuna, lime juice, red onion, cilantro, jalapeño pepper (if using), avocado, and mango.
Season with salt and pepper to taste.
Cover and refrigerate for at least 30 minutes, or up to 4 hours.
Serve with tortilla chips or crackers.
Video
Notes
TipsFor the best flavor, use fresh tuna that is sashimi grade.If you don't have jalapeño peppers, you can substitute another type of chili pepper, such as serrano or habanero peppers.Be careful not to overcook the tuna. The lime juice will start to cook the tuna after about 30 minutes, so it is important to refrigerate the ceviche for at least that long.Ceviche is best served immediately, but it can also be stored in the refrigerator for up to 24 hours.VariationsYou can add other vegetables to the ceviche, such as cucumbers, tomatoes, or bell peppers.You can also add different fruits to the ceviche, such as pineapple, papaya, or grapefruit.If you want a spicier ceviche, you can add more jalapeño pepper or other chili peppers.You can also add a bit of sweetness to the ceviche by adding a teaspoon of honey or agave nectar.