Pat the salmon fillets dry with a paper towel and season both sides with salt and freshly ground black pepper.
In a large skillet, heat the olive oil over medium-high heat. Add the salmon fillets, skin-side down if using skin-on fillets, and cook for 4-5 minutes per side, until golden brown and cooked through. Transfer the salmon to a plate and set aside.
In the same skillet, add the chopped onion and sauté for 3-4 minutes, until softened and translucent. Add the minced garlic and cook for an additional 30 seconds, until fragrant.
Stir in the chopped sun-dried tomatoes (or cherry tomatoes), and cook for another 2 minutes to soften and release their flavors.
Add the baby spinach to the skillet, and cook for 1-2 minutes until wilted.
If using white wine, pour it into the skillet, and let it reduce by half, about 2 minutes. If not using wine, proceed to the next step.
Pour in the chicken or vegetable broth and bring the mixture to a simmer. Cook for 2-3 minutes, until the broth has reduced by half.
Add the heavy cream to the skillet, stirring to combine. Simmer the sauce for 4-5 minutes, or until it thickens slightly.
Stir in the grated Parmesan cheese, allowing it to melt into the sauce.
Add the chopped parsley, lemon juice (if using), and Italian seasoning. Stir to combine and adjust the seasoning with salt and pepper if necessary.
Return the cooked salmon fillets to the skillet, nestling them into the sauce. Spoon some sauce over the salmon and cook for another 2-3 minutes, or until the salmon is heated through.
Serve the Tuscan salmon over pasta, rice, or with crusty bread to soak up the delicious sun-dried tomato sauce. Garnish with additional chopped parsley, if desired.