Pat steak strips dry and toss with Cajun seasoning until evenly coated. Bring a large pot of salted water to a boil for the pasta.

Heat olive oil in a large skillet over medium-high heat until shimmering. Add steak in a single layer and sear undisturbed for 2-3 minutes until well-browned. Flip and cook 1-2 minutes more for medium-rare. Transfer to a plate and tent with foil. The steak will finish cooking in the sauce later.

Reduce heat to medium. Add butter to the same skillet. Once melted, add onion and cook 4-5 minutes until softened and golden at the edges. Add garlic and cook 30 seconds until fragrant. Pour in chicken broth and scrape up any browned bits from the pan.

While the onion cooks, add penne to the boiling water and cook 1 minute less than package directions for al dente. Reserve 1/2 cup pasta water, then drain.

Stir heavy cream into the skillet and bring to a gentle simmer. Cook 3-4 minutes until slightly thickened. Whisk in Parmesan until melted and smooth. Add half the reserved pasta water and simmer 2 minutes more. The sauce should coat the back of a spoon.

Add drained penne and any accumulated steak juices to the sauce, tossing to coat. Nestle steak strips on top and simmer together 1-2 minutes until steak is warmed through and pasta is perfectly tender. Add remaining pasta water if needed to loosen. Taste and adjust seasoning with more Cajun spice or salt.

Divide among bowls, shower with extra Parmesan and chopped parsley. Serve immediately with crusty bread to mop up every drop of sauce.