1. Preheat oven to 450°.
2. Sprinkle salmon with 1/2 teaspoon salt and 1/8 teaspoon pepper. Heat a large cast-iron skillet over high heat. Add 2 teaspoons oil; swirl. Add fillets, skin side down; cook for 3 minutes or until skin begins to brown, gently pressing fillets. Place pan in oven. Bake at 450° for 6 minutes or until desired degree of doneness.
3. Heat a nonstick skillet over medium-high heat. Add 1 teaspoon oil; swirl. Add tomatoes; sauté 1 minute. Add garlic; sauté for 30 seconds, stirring constantly. Add spinach; remove from heat. Toss until spinach wilts. Stir in 1/4 teaspoon salt and 1/8 teaspoon pepper. Place about 1/2 cup spinach mixture on each of 4 plates; top each serving with 1 fillet and basil.