Bring a large pot of salted water to a boil, then add the pasta shells. Cook until al dente, about 8–10 minutes. Reserve ¼ cup of pasta water, drain, and set aside.

Pat the steak cubes dry and toss with Cajun seasoning, salt, and black pepper. Let sit while heating the skillet for searing.

Heat the skillet over medium-high heat with olive oil. Sear the steak bites in batches for 2–3 minutes until golden. Add butter and garlic in the last minute.

Reduce heat to medium. Add another tablespoon of butter and the remaining garlic. Sauté for 1 minute, then add heavy cream to deglaze.

Stir in cheddar and Parmesan cheese until melted. Adjust consistency with reserved pasta water if needed.

Fold in the cooked pasta shells to the sauce, top with seared steak, and garnish with parsley.