1. Sear the Salmon:

1. Heat the olive oil in a large skillet over medium-high heat.
2. Season the salmon fillets on both sides with salt and pepper.
3. Place the salmon fillets in the hot skillet, flesh-side down first (if using skin-on fillets, place skin-side down first).
4. Sear for 5 minutes on each side, or until cooked to your liking. The internal temperature should reach 145°F (63°C) for medium doneness.
5. Remove the salmon from the pan and set aside.

2. Make the Sauce:

1. Reduce the heat to medium.
2. Add the butter to the same skillet (with the remaining juices from the salmon).
3. Once the butter is melted, add the diced garlic and cook for about 1 minute, or until fragrant. Be careful not to burn the garlic.
4. Add the diced onion and cook until softened and translucent, about 3-5 minutes.
5. Pour in the white wine (if using) and allow it to reduce slightly, scraping up any browned bits from the bottom of the pan (about 2 minutes).
6. Add the drained sun-dried tomatoes and cook for 1-2 minutes, stirring occasionally, to release their flavors.
7. Reduce the heat to low.
8. Pour in the half-and-half (or heavy cream/milk mixture).
9. Bring the sauce to a gentle simmer, stirring occasionally.
10. Season with salt and pepper to taste.
11. Add the baby spinach leaves and stir until wilted (about 1-2 minutes).
12. Stir in the grated Parmesan cheese until melted and the sauce is smooth.
13. Optional: Thicken the Sauce: If you prefer a thicker sauce, whisk together the cornstarch and water in a small bowl to create a slurry. Add the slurry to the center of the simmering sauce and whisk quickly to incorporate. Continue to simmer, stirring constantly, until the sauce thickens to your desired consistency.

3. Combine and Serve:

1. Gently return the seared salmon fillets to the pan, nestling them into the sauce.
2. Spoon the sauce over the salmon fillets.
3. Sprinkle with chopped fresh parsley.
4. Serve immediately over pasta, rice, mashed potatoes, or steamed vegetables.