Bring a large pot of salted water to a rolling boil. Add cheese tortellini and cook according to package directions until al dente, stirring occasionally. Before draining, scoop out 1/2 cup of pasta water into a heatproof measuring cup. Drain tortellini in a colander and set aside.

While pasta cooks, pat steak cubes completely dry with paper towels. Season evenly with Cajun seasoning, smoked paprika, garlic powder, salt, and black pepper. Heat your 12-inch skillet while you season the steak (2-3 minutes).

Heat olive oil in skillet over medium-high heat until shimmering. Add steak bites in a single layer without overcrowding (work in batches if needed). Cook for 2-3 minutes per side for medium doneness on 1-inch cubes, until deeply browned with a caramelized crust. Transfer to a plate to rest for 3-5 minutes while you make the sauce.

Reduce heat to medium. Add butter to the same skillet. Once melted, add minced garlic and cook for 1 minute, stirring constantly, until fragrant but not browned. Stir in red pepper flakes, Dijon mustard, and lemon juice.

Add chopped parsley and chives to the butter mixture. Return cooked tortellini and rested steak bites to the skillet, tossing gently to coat.

Gradually add reserved pasta water, a few tablespoons at a time, until sauce coats the back of a spoon and looks creamy. Sprinkle in Parmesan cheese and stir until melted. Taste and adjust seasoning if needed. Serve immediately, garnished with extra Parmesan and fresh parsley.