Step 1: Prep the Steak and Seasoning
If making your own Cajun seasoning, combine all spice ingredients in a small bowl. Set aside ½ teaspoon for later use in the sauce. Place the thinly sliced steak in a medium bowl and toss with 1 tablespoon of the Cajun seasoning mix until evenly coated. Let it sit at room temperature for 15-20 minutes. Bring a large pot of salted water to a boil for the pasta.

Step 2: Sear the Steak
Heat the olive oil in a large, deep skillet or Dutch oven over medium-high heat until shimmering. Working in batches to avoid overcrowding, add the seasoned steak slices in a single layer. Sear for 1-2 minutes per side, just until browned and cooked to your desired doneness (it will cook slightly more later). Remove the steak to a clean plate and set aside. Do not wipe out the skillet.

Step 3: Cook the Pasta
Add the pasta to the boiling water and cook according to package directions until al dente. Before draining, reserve 1 cup of the starchy pasta water. Drain the pasta and set aside.

Step 4: Build the Flavor Base – The “Holy Trinity”
In the same skillet used for the steak, reduce heat to medium. Add the butter. Once melted, add the diced onion, bell pepper, and celery (the “holy trinity”). Sauté for 6-8 minutes, until the vegetables have softened and the onions are translucent. Add the minced garlic and cook for 1 more minute until fragrant.

Step 5: Create the Sauce Foundation
Push the vegetables to the side of the pan. Add the tomato paste to the center and let it cook for 1 minute, stirring, to deepen its flavor. Sprinkle the reserved ½ teaspoon of Cajun seasoning over the vegetables and tomato paste and stir everything together. Pour in the chicken broth to deglaze the pan, scraping up all the browned bits (fond) from the bottom. Let it simmer for 2-3 minutes to reduce slightly.

Step 6: Make the Sauce Creamy
Reduce the heat to medium-low. Pour in the heavy cream and stir to combine. Bring the sauce to a gentle simmer (do not boil vigorously). Let it cook for 3-4 minutes, stirring occasionally, until it begins to thicken slightly. Stir in the grated Parmesan cheese until it is fully melted and the sauce is smooth.

Step 7: Combine Pasta, Sauce, and Steak
Add the drained pasta and the seared steak (along with any accumulated juices) to the skillet with the sauce. Using tongs, toss everything together vigorously for 1-2 minutes, until the pasta is evenly coated and everything is heated through. If the sauce seems too thick, add a splash of the reserved pasta water, a little at a time, until it reaches your desired consistency. The sauce should cling to the pasta beautifully. Taste and adjust seasoning with extra salt, black pepper, or a pinch more Cajun seasoning if needed. Stir in a teaspoon of lemon juice if using, for a touch of brightness.

Step 8: Garnish and Serve Immediately
Remove from heat. Garnish generously with chopped fresh parsley and an extra sprinkle of Parmesan cheese. Serve immediately in warm bowls, with crusty bread on the side to soak up every last drop of the incredible creamy Cajun sauce.