Bring the salmon to room temperature for about 15 minutes before cooking so it sears evenly. Pat it dry and season both sides well with salt and pepper. Drying the salmon helps it form a golden crust instead of steaming.
Heat your skillet over medium-high heat and add the oil. When it’s shimmering, place the salmon flesh-side down and press gently so it touches the pan fully. Cook about five minutes per side until golden and crisp.
Remove the salmon and add butter to the same pan. Once melted, stir in garlic for about 30 seconds, then add diced onion until it turns soft and translucent. These aromatics create the base of the creamy sauce.
Pour in the white wine (or stock) and scrape the pan to release all the flavorful browned bits. Add red pepper flakes and salt. Stir in the cherry tomatoes and lightly press them so they burst. Add spinach and cook until wilted.
Pour in the coconut milk and let it simmer for 3–4 minutes to thicken. Add lemon juice and basil. Return the salmon to the pan and spoon the sauce over it. Remove from heat and serve warm.