Heat a large grill pan over medium-high heat.
Sprinkle fish with 1/4 teaspoon salt. Combine the sugar and 1 1/2 tablespoons juice; spread evenly over fish. Coat pan with cooking spray. Add fish to pan, and cook 3 minutes on each side or until desired degree of doneness.
While fish cooks, combine remaining 1/4 teaspoon salt, 1 1/2 tablespoons juice, tomato, cilantro, avocado, and jalapeño, tossing gently. Serve fish with salsa.