1. Combine the onion, lime juice, 1/4 teaspoon salt, and avocados.
2. Heat a grill pan over medium-high heat; coat pan with cooking spray. Sprinkle tuna with remaining 1/2 teaspoon salt. Add tuna to pan; cook 4 minutes on each side or to desired degree of doneness. Cut tuna into 1/4-inch-thick slices. Warm tortillas according to the package directions. Divide avocado mixture evenly among tortillas. Divide tuna evenly among tortillas.