In a small bowl, combine the spices for the cajun seasoning.
In a medium bowl, combine shrimp, 2 1/2 tsps of cajun seasoning, and 1 tablespoon olive oil. Toss to coat and let sit for 10-15 minutes.
Bring a large pot of water to boil and add pasta. Cook until al dente. Reserve 1/3 cup pasta water. Drain pasta and set aside.
In a large skillet, heat 1 1/2 tablespoons of butter. Saute shrimp in small batches 3-4 minutes per side until cooked through. Remove shrimp to plate.
Add 1 tablespoon butter to now empty skillet and saute onions. Add spinach and wilt gently. Add 1 teaspoon of cajun seasoning to vegetables. Add pasta water to deglaze pan. Transfer vegetables to plate with shrimp
Melt remaining butter in skillet. Add half and half and simmer to reduce by 1/4 to 1/3. Add cheese and stir together until somewhat thickened.
Add pasta to skillet and toss to coat. Add shrimp and veggies and coat evenly with sauce. Top with fresh parsley and additional cheese