Cut your salmon fillets into large cubes, then sprinkle 2 tablespoons Cajun seasoning, salt, and pepper over them, gently rubbing to coat evenly. This helps the seasoning cling and gives each bite a spicy kick.
In a large pan over medium heat melt 1 tablespoon butter and cook the salmon cubes for 2 to 3 minutes per side until they look opaque and edges start to brown. You'll see small golden bits form around each cube, which add great flavor.
Move the salmon to a plate, then add minced garlic to the pan and sauté until it turns translucent and you smell its sweet aroma, about 1 minute. This step infuses the base of your sauce with garlic flavor.
Add baby spinach leaves to the pan and cook for a few minutes, stirring, until they’re wilted and no longer feel soggy. This keeps the spinach bright and tender without turning it watery.
Meanwhile bring a large pot of water to a boil and cook the fettuccine following package instructions until al dente, then drain. Al dente pasta holds up best against the creamy sauce without getting mushy.
Lower the heat and toss in ¼ cup butter, 1 cup heavy cream, the remaining Cajun seasoning, salt, and Parmesan cheese, stirring until the cheese melts and the sauce thickens slightly. Keep the heat gentle so the cream doesn’t break and you end up with a velvety texture.
Add the drained pasta and salmon cubes back into the pan, stirring gently to coat everything in the sauce. Finish with chopped parsley and a squeeze of lemon juice to brighten the flavors before serving.