Season and rest the steak: Pat the sirloin completely dry, then rub it with 1 tablespoon Cajun seasoning and the kosher salt. Let it sit at room temperature for 15 minutes while you prep everything else. Uneven, fridge-cold steak cooks unevenly.
Boil the pasta: Cook the rigatoni in well-salted water to just under al dente about 1 minute less than the package says. Scoop out 1 cup of pasta water before draining. You’ll use most of it, and running out is annoying. Pro tip: set the measuring cup in the sink next to the colander so you don’t reflexively dump the whole pot.
Sear the steak: Heat the olive oil in the skillet over medium-high until it shimmers. Lay the steak down and don’t move it for 3 to 4 minutes, until a deep brown crust forms. Flip and cook another 3 to 4 minutes, pulling it at 130–135°F for medium-rare. Transfer to a board and let it rest while you build the sauce.
Sauté the vegetables: Drop the heat to medium and add the butter to the same pan, scraping up the browned bits. Add the onion and bell pepper and cook for 4 minutes, until softened and just starting to color at the edges. Stir in the garlic and cook 30 seconds, until fragrant.
Bloom the spices in the fat before any liquid: Add the Cajun seasoning, smoked paprika, cayenne, and tomato paste directly to the hot fat and stir constantly for 30 to 45 seconds, until the mixture darkens a shade and smells toasty rather than powdery. This matters more than it sounds like it should. The flavor compounds in paprika and dried chilies are fat-soluble, so they release into oil but not into cream or broth. Dump the spices into liquid instead and you get a sauce that tastes flat and faintly chalky no matter how much you add.
Build the sauce: Pour in the chicken broth and let it bubble for a minute, then add the heavy cream. Simmer gently for 3 to 4 minutes until it thickens enough to coat the back of a spoon. Take the pan off direct heat before stirring in the Parmesan so it melts smoothly instead of seizing.
Combine: Add the drained pasta and toss, splashing in reserved pasta water a few tablespoons at a time until the sauce loosens and clings. Stir in the lemon juice it won’t taste lemony, it just stops the cream from feeling heavy.
Slice and finish: Cut the rested steak against the grain into thin slices, then fold them in along with any juices that pooled on the board. Top with parsley, green onion, and a grind of black pepper. Serve right away, while the sauce is still glossy.
