Boil the pasta. Bring a large pot of salted water to a boil. Cook the pasta until just al dente, 1 minute shy of the package instructions. Drain and set aside. Toss with a drizzle of oil to prevent sticking.

Season the shrimp. Pat shrimp very dry. Toss with 1 tablespoon Cajun seasoning, a pinch of salt, and black pepper. If your Cajun blend is salty, skip extra salt.

Sear the shrimp. Heat a large skillet over medium-high. Add olive oil. Cook shrimp in a single layer 1–2 minutes per side until opaque and lightly charred at the edges. Transfer to a plate. Do not overcook.

Start the roux. In the same skillet over medium heat, add butter. When melted, stir in flour and cook 60–90 seconds, whisking, until foamy and lightly golden. This prevents a raw flour taste.

Build the sauce. Slowly whisk in milk and chicken broth. Simmer 2–3 minutes until slightly thickened. Stir in smoked paprika, a pinch of black pepper, and lemon zest. Reduce heat to low.

Add the cheese. Remove the pan from heat. Gradually whisk in cheddar and Parmesan by the handful until melted and smooth. Return to low heat only if needed. Avoid boiling.

Boost with yogurt. Stir in Greek yogurt until the sauce is silky. Taste and adjust with salt, pepper, or more Cajun seasoning for heat. If sauce is too thick, splash in extra milk.

Combine. Fold in cooked pasta and most of the shrimp, reserving a few for topping. Gently toss to coat every piece. If using breadcrumbs, preheat the broiler now.

Optional crunchy top. Transfer to a broiler-safe dish. Sprinkle with panko and a light dusting of Cajun seasoning. Broil 1–3 minutes until golden. Watch closely.

Finish and serve. Top with reserved shrimp, sliced green onions, and a squeeze of lemon. Serve hot with extra Cajun seasoning on the side.