1. Preheat oven to 400°.
2. Combine 2 tablespoons lime juice, yogurt, 1 tablespoon water, 1/4 teaspoon salt, and avocado in a small bowl; mash with a fork until smooth.
3. Heat a large ovenproof skillet over high heat. Add 1 1/2 tablespoons oil to pan; swirl to coat. Sprinkle the fish evenly with remaining 1/2 teaspoon salt and black pepper. Add fish to pan, skin side down, and sauté for 2 minutes or until lightly browned. Place pan in oven; cook at 400° for 8 minutes or until desired degree of doneness.
4. Combine remaining 1 tablespoon lime juice and remaining 1 1/2 tablespoons olive oil in a medium bowl, stirring with a whisk. Add cilantro and onion; toss gently to coat. Spoon about 3 tablespoons avocado mixture on each of 4 plates. Top each serving with 1 fillet and 1/2 cup salad. Serve with Tabasco, if desired.