Start by patting your salmon fillets dry with paper towels. This helps them sear nicely. Season both sides generously with Cajun seasoning and smoked paprika.
In a large skillet, heat the olive oil over medium-high heat. Once hot, add the salmon, skin side down if applicable. Sear for about 4-5 minutes on each side until golden brown and cooked through. Remove the salmon from the skillet and set aside on a plate.
In the same skillet, melt the butter. Add the finely diced onion and red bell pepper. Sauté for about 3-4 minutes, or until the vegetables are soft. Add the minced garlic and cook for another minute until fragrant.
Pour in the chicken broth, scraping up any browned bits from the bottom of the skillet. Allow it to simmer for a couple of minutes before adding in the heavy cream, crushed red pepper flakes, black pepper, and additional Cajun seasoning. Stir well to combine.
If you prefer a thicker sauce, mix the cornstarch with a little water to create a slurry and add it to the sauce while stirring continuously. Bring it to a gentle simmer until it thickens.
Remove the skillet from heat and stir in the freshly grated Parmesan cheese and lemon juice until melted and combined.
Return the salmon to the skillet, spooning the creamy sauce over the top. Let it warm through for a minute, then garnish with sliced green onions and chopped parsley.