Prep the fish: Pat cod fillets dry with paper towels. Season both sides with salt, pepper, and a pinch of paprika.
Heat the pan: Place a large non-stick or cast-iron skillet over medium-high heat. Add olive oil and let it heat until shimmering.
Cook the cod: Add the fillets gently to the pan. Cook for 3–4 minutes without moving them to build a golden crust.
Flip and finish: Turn the fillets carefully. Add the butter and spoon it over the fish as it cooks another 3–4 minutes until opaque and flaky.
Make the salsa: In a bowl, mix tomatoes, onion, garlic, jalapeño, lime juice, basil, and salt. Toss until well combined.
Serve: Place cod on plates and top generously with fresh salsa.