Mix the lime juice, garlic powder, a pinch of black pepper and a large pinch of coarse salt together in a non-metal bowl.
Then add the tuna and mix gently until well combined.
Add the mango, red onion, cilantro and avocado if using, and mix gently.
Then cover and let sit in the refrigerator for at least 1 hour.
Give everything a gentle mix before serving.
I like to serve this in martini glasses. Serve with tostadas, tortilla chips, pita chips, crackers, or rice crackers. Or, you could serve these in wonton cups (recipe below).