Heat olive oil in a large skillet over medium-high heat. Add shrimp and sprinkle with Cajun seasoning. Sauté for 2–3 minutes until shrimp turn pink. Remove from pan and set aside.
In the same pan, add butter and diced onion. Sauté until the onion becomes translucent, about 3 minutes.
Add minced garlic and cook for 30 seconds until fragrant.
Pour in heavy cream and stir in Parmesan cheese. Reduce heat and let the sauce simmer for 3–4 minutes until slightly thickened.
Add broccoli florets to the pan, cover, and steam for 4–5 minutes until tender.
Return the shrimp to the skillet and stir to coat them in the sauce. Simmer for another 2 minutes.
Season with salt and pepper as needed.
Serve hot over cooked rice or cauliflower rice and garnish with chopped green onions.