Prep the shrimp and sausage. Pat the shrimp dry with paper towels so they sear, not steam.
Toss with 1 tablespoon Cajun seasoning and a pinch of salt. Slice the sausage into 1/4-inch rounds.
Heat the pan. Set a large skillet over medium-high heat and add 1 tablespoon olive oil. When it shimmers, add the sausage in a single layer.
Brown the sausage. Cook 3–4 minutes, flipping once, until the edges get crisp and browned.
Transfer to a plate and leave the drippings in the skillet.
Sauté the veggies. Add the remaining tablespoon of oil, then the bell peppers and red onion. Season with a pinch of salt and the remaining 1 tablespoon Cajun seasoning. Cook 4–5 minutes, stirring occasionally, until slightly tender with some char.
Add garlic and spices. Stir in the minced garlic, smoked paprika, and cayenne (if using).
Cook 30 seconds until fragrant.
Cook the shrimp. Push the veggies to the sides and add the shrimp to the center. Cook 1–2 minutes per side, just until pink and opaque.
Bring it together. Return the sausage to the skillet. Add the butter and lemon juice.
Toss everything until glossy and well coated. Taste and adjust salt and pepper.
Finish and serve. Sprinkle with chopped parsley or green onions. Serve hot over rice, spoon into bowls, or pair with warm bread to soak up the juices.