Bring a large pot of salted water to a rolling boil. Add the pasta and cook until just al dente, about 8–10 minutes. Drain and set aside, reserving about ½ cup of pasta water.
While the pasta cooks, peel and devein the shrimp if needed. Pat them dry thoroughly with paper towels to ensure a good sear. Toss the shrimp with a teaspoon of Cajun seasoning and a pinch of salt.
Heat the skillet over medium-high heat and add a tablespoon of oil. Once shimmering, add the seasoned shrimp in a single layer. Sear for 2–3 minutes on each side until they turn bright pink and develop a slight char. Remove the shrimp from the pan and set aside.
In the same skillet, add the minced garlic and cook for about 30 seconds until fragrant, with a pungent aroma filling the air. Stir in the remaining Cajun seasoning and cook for another 30 seconds to bloom the spices.
Pour in the heavy cream, scraping the bottom of the pan to lift any browned bits. Bring the cream to a gentle simmer and let it thicken slightly, about 2–3 minutes, until the sauce is velvety and coats the back of a spoon.
Add the cooked pasta to the skillet, tossing to coat it evenly with the spicy, creamy sauce. Squeeze the juice of half the lemon over the pasta, stirring to combine. If the sauce is too thick, stir in a bit of the reserved pasta water until silky and well-coated.
Return the seared shrimp to the skillet and toss everything together gently. Add the butter, allowing it to melt into the sauce, adding richness and shine. Taste and adjust seasoning with salt, pepper, or more lemon juice as needed.
Cook for another minute to allow flavors to meld and ensure everything is heated through. The sauce should be thick and glossy, with the shrimp nestled atop the pasta. Turn off the heat and serve immediately, garnished with extra lemon wedges if desired.
