Heat the olive oil in a large skillet over medium-high heat. Season the salmon fillets on both sides with salt and pepper. Sear the salmon in the hot pan, flesh-side down first, for 5 minutes on each side, or until cooked to your liking. Once cooked, remove the salmon from the pan and set it aside.
In the same pan, melt the butter in the remaining juices. Add the garlic and fry until fragrant, about 1 minute. Add the diced onion and cook until softened.
Stir in the sun-dried tomatoes and cook for 1-2 minutes to release their flavors.
Reduce the heat to low, then pour in the half-and-half (or heavy cream). Bring the mixture to a gentle simmer, stirring occasionally. Season with salt and pepper to taste.
Add the spinach leaves and allow them to wilt in the sauce. Stir in the Parmesan cheese and let the sauce simmer for another minute until the cheese has melted and the sauce has thickened slightly. For a thicker sauce, add the cornstarch mixture and continue to simmer while stirring until the sauce thickens.
Return the salmon to the pan, sprinkle with fresh parsley, and spoon the sauce over each fillet.
Serve the creamy Tuscan salmon over pasta, rice, or steamed vegetables.