Add pasta to a large pot of salted boiling water and let cook according to package instructions, until just al dente. Drain well, reserving about ½ cup of the pasta water.
While pasta is cooking, pat salmon dry and season with about 1 Tbsp of Cajun seasoning.
Add 1 Tbsp olive oil to a large skillet over medium heat. When hot, add salmon skin-side up (if skin is on salmon) and cook for 3-5 minutes. Gently flip and cook for an additional 3-4 minutes, until salmon is flaky. Move to a plate and set aside.
Add remaining Tbsp oil to the skillet. When hot, add onions and red pepper. Cook for about 4 minutes until veggies are softening. Add garlic, remaining 1 Tbsp cajun seasoning, and salt and cook for another minute.
Add the undrained diced fire-roasted tomatoes to skillet, along with ¼ cup of the reserved pasta water and bring to a simmer. Use a wooden spoon to scrape up any brown bits from the bottom of the pan. Let cook, stirring occasionally, for about 3 minutes to reduce liquid.
Add heavy cream to skillet and allow the sauce to return to a gentle simmer. Simmer for about 3 minutes to let sauce thicken.
Add pasta to skillet, along with scallions and parsley and toss to coat in sauce. Flake salmon into bite-sized pieces and gently toss with pasta. If needed, add in additional ¼ cup pasta water to thin out sauce.
Serve Cajun salmon pasta topped with additional chopped parsley and scallions, if desired.