1. Preheat oven to 400°.
2. Heat a large ovenproof skillet over high heat. Add 1 tablespoon oil to pan; swirl to coat. Sprinkle fillets with 1/2 teaspoon salt and 1/4 teaspoon pepper. Add fillets, flesh side down, to pan, and sauté for 2 minutes. Place pan in oven; cook at 400° for 3 minutes or until desired degree of doneness.
3. Heat a large cast-iron skillet over medium heat. Add remaining 2 tablespoons oil to pan; swirl to coat. Add garlic, and cook 2 minutes or until lightly browned, stirring occasionally. Increase heat to medium-high. Add tomatoes to pan; sauté for 2 minutes or until skins blister, stirring frequently. Remove pan from heat. Sprinkle tomato mixture with the remaining 1/4 teaspoon salt, remaining 1/4 teaspoon black pepper, basil, and shallots; toss to combine. Serve with fish.