Slice the avocado lengthwise around the pit, twist to separate halves, remove the pit, and scoop the flesh into a medium mixing bowl.
Add the kosher salt, lime juice, and garlic powder to the avocado and mash with a fork until creamy and mostly smooth.
Stir in the minced red onion and minced jalapeño, if using, until evenly combined.
Fold in the tuna until it is distributed evenly through the avocado mixture.
Serve immediately on bread, in a wrap or lettuce leaf, with crackers, or enjoy on its own.