Pat salmon dry and season with salt and pepper. Heat olive oil over medium-high heat in a large nonstick skillet.Add salmon skin side down. Cook for about 4 minutes or until skin is crispy. Flip over and cook another 2 to 3 minutes, or until cooked to your liking. Remove from skillet and set aside.
Add the butter to the skillet and cook the onion for 3 minutes to soften. Add garlic and cook another minute.
Add wine and cook for 30 seconds.
Add heavy cream, basil, salt, black pepper and red pepper flakes. Simmer for 2 to 3 minutes.
Add tomatoes. Continue to simmer until heavy cream sauce has thickened some.
Stir in Parmesan cheese and spinach and stir until cheese is melted and spinach is wilted.Check for seasoning and add more salt if desired.