Bring a large pot of salted water to a boil and cook pasta shells until al dente; reserve ¼ cup pasta water, drain and set aside.
Pat steak cubes dry and toss with Cajun seasoning, salt, and pepper.
Heat olive oil in a cast iron skillet over medium-high heat, then sear steak bites in batches for 2–3 minutes per side.
In the last minute of cooking steak, add 2 tablespoons butter and 2 minced garlic cloves, tossing to coat the steak.
Remove steak bites from skillet and set aside.
Reduce skillet heat to medium, add 2 tablespoons butter and 3 minced garlic cloves, and sauté for 1 minute.
Pour in heavy cream and stir to deglaze the pan, then simmer gently.
Add shredded cheddar, Parmesan, paprika, black pepper, and salt; stir until cheese melts and sauce is creamy.
Toss cooked pasta shells into cheese sauce, adding reserved pasta water if needed to loosen the sauce.
Top cheesy shells with seared steak bites, garnish with chopped parsley, and serve immediately.