Cook pasta according to package directions. Reserve 1 cup of pasta water, drain pasta and set aside.
Prep salmon:
Pat dry salmon fillets with a paper towel and season with salt and pepper on both sides.
Cook veggies:
In a large skillet, heat oil and butter over medium-high heat. Add the onions and cook for about 4 minutes until the onions are golden. Add the garlic and stir, add the tomatoes and cook until the mixture simmers, about 3 to 4 minutes. Remove from the pan onto a large plate.
Cook salmon:
Add a little more oil to the pan, and cook the salmon, 4 to 6 minutes per side until golden/seared on each side and the meat separates easily with a fork. Remove from the pan onto the plate.
Make the sauce:
Add the cream to the pan and ½ cup of the reserved pasta water. Bring it to a simmer and with a wooden spatula, scrape the brown bits from the bottom of the pan. Add the Cajun seasoning, and mix to incorporate with the sauce.
Combine:
Break up the salmon into large pieces and add them into the pan along with the tomatoes, onions, and chopped spinach. Toss the cooked pasta in, mix to coat everything with the sauce. Add remaining pasta water, a little bit at a time, until you reach the thickness and consistency of sauce you like.
Serve:
Serve with chopped parsley and parmesan cheese, if desired.
Notes
You can swap heavy cream or half & half for pasta water to lighten up this dish, in the same amount.