Preheat the oven to 425°F (220°C).
Arrange the sliced beef sausage, onion, and bell pepper on a large sheet pan. Drizzle with olive oil and sprinkle with 1 teaspoon Cajun seasoning. Toss until evenly coated.
Roast for 10 minutes.
Remove the pan from the oven and stir in the tomato paste until the vegetables are evenly coated.
Add the frozen cauliflower rice and the remaining Cajun seasoning. Mix everything thoroughly.
Return the pan to the oven and roast for 15–20 minutes, stirring once or twice, until the cauliflower rice is tender and excess moisture has evaporated.
Taste the Sheet Pan Jambalaya and season with sea salt and black pepper as needed.
Arrange the shrimp over the top and drizzle with melted butter.
Bake for another 5–7 minutes until the shrimp turn pink and are fully cooked.
Garnish with fresh parsley and a squeeze of lemon juice before serving.