Season the salmon. Pat salmon fillets dry with a paper towel, then season both sides generously with kosher salt and freshly ground black pepper.
Sear the fillets. Heat 1 Tbsp olive oil and 1 Tbsp oil from the sun-dried tomato jar in a large skillet over medium-high heat. When the oil shimmers, add the salmon skin-side down. Cook for 4-5 minutes, pressing gently so the skin stays flat. Use tongs or a fish spatula to carefully flip, then cook for 2-3 minutes more, just until the top turns opaque. Transfer to a plate.
Sauté aromatics. Reduce the heat to medium, then melt 2 Tbsp butter in the same pan. Add 3 cloves minced garlic, 1 tsp dried Italian seasoning, and a generous pinch of red pepper flakes. Cook, stirring constantly, until fragrant, about 30-60 seconds. Add 1½ cups of halved cherry tomatoes, season with kosher salt, then cook until the tomatoes begin to burst, about 3-4 minutes.
Deglaze and reduce. Pour in ¼ cup dry white wine, then use a spatula to scrape up any browned bits. Cook for about 1 minute, or until the liquid has mostly evaporated.
Make the sauce. Stir in ½ cup heavy cream, ⅓ cup mascarpone, ⅓ cup of grated Parmesan, and ⅓ cup of sun-dried tomatoes. Season with a pinch of kosher salt and several grinds of black pepper. Let the sauce bubble gently for 2-3 minutes, until slightly thickened.
Add spinach. Fold in 3 packed cups of baby spinach and ¼ cup chopped fresh basil. Stir just until the spinach wilts. Add the juice from ½ a medium lemon, then stir well. Taste and adjust the sauce if needed.
Finish the sauce. Return the salmon (skin-side up, if using skin-on) to the skillet. Spoon the sauce on top.
Serve. Plate the salmon, then ladle the creamy Tuscan sauce on top and garnish with extra basil, lemon wedges, and black pepper. Enjoy immediately with your favorite pasta, crusty bread, or lemony mashed potatoes.