Dice onion; mash avocados. Add to a mixing bowl along with lime juice and 1/8 teaspoon salt; mix together.
Spray a skillet with cooking spray and heat over medium. Season fish with remaining salt and then add to hot skillet. Cook for 3-4 minutes on each side until flaky. Remove from heat and cut into smaller pieces.
Warm tortillas. Cut tomato in half, remove seeds, then chop.
Add fish and avocado mixture to each tortilla. Top with tomatoes, and serve with pineapple chunks on the side.