Boil salted water and cook penne according to package directions. Reserve 1 cup pasta water before draining.
Pat salmon dry and season both sides with salt, pepper, and 1 Tbsp Cajun seasoning.
Heat butter and olive oil in large skillet over medium heat. Add salmon skin-side down and cook 4 minutes without moving.
Flip salmon and cook 4-6 minutes until it flakes easily and reaches 145F. Remove to a plate.
Add onions and bell peppers to the same pan and cook 4 minutes until softened.
Add garlic and cook 2 minutes, stirring constantly. Season with salt and pepper.
Stir in tomato paste and cook 3-4 minutes until darkened.
Turn heat to low and add remaining Cajun seasoning. Pour in heavy cream slowly, stirring constantly.
Simmer 5 minutes until thickened.
Remove from heat. Add spinach and Parmesan, stirring until wilted and melted.
Stir in lemon juice and zest, then add 1/2 cup reserved pasta water to thin sauce.
Add cooked penne to sauce and toss to coat.
Break salmon into large chunks and gently fold into pasta.
Taste and adjust seasoning. Garnish with parsley before serving.