Heat half of the reserved oil from the sun-dried tomatoes in a large skillet over medium-high heat.
Season the salmon fillets lightly with salt and black pepper. Place them in the hot skillet and cook for about 1 to 2 minutes per side until lightly golden. The salmon does not need to be fully cooked at this stage. Transfer it to a plate.
Add the remaining reserved oil to the same skillet.
Stir in the chopped onion and crushed garlic. Cook for about 5 minutes, stirring occasionally, until the onion becomes soft and fragrant.
Pour in the cooking cream along with one-third cup of water. Stir well and bring the sauce to a gentle boil.
Add the chopped sun-dried tomatoes and stir until evenly combined.
Return the salmon fillets to the skillet, nestling them into the creamy sauce.
Reduce the heat to medium-low and simmer for about 3 to 5 minutes, or until the salmon is almost cooked through.
Add the baby spinach and capers, stirring gently until the spinach wilts into the sauce.
Scatter the fresh basil leaves over the top just before serving.
Serve the One-Pan Creamy Tuscan Salmon with crusty bread, mashed potatoes, rice, or buttered pasta to soak up the delicious creamy sauce.