Bring a large pot of salted water to a boil and cook tortellini until tender but firm; reserve ½ cup pasta water, then drain.
Pat steak cubes dry and season with Cajun seasoning, smoked paprika, garlic powder, salt, and pepper.
Heat olive oil in a large skillet over medium-high heat and sear steak bites for 2–3 minutes per side until browned; set aside.
Reduce heat to medium; melt butter in skillet, add minced garlic and cook until fragrant, about 1 minute.
Stir in red pepper flakes, Dijon mustard, lemon juice, parsley, and chives; mix well.
Return tortellini and steak bites to the skillet and toss with the cowboy butter sauce.
Add reserved pasta water as needed to achieve a smooth sauce and stir in Parmesan cheese until melted.
Serve hot, garnished with extra Parmesan, fresh parsley, and a sprinkle of Cajun seasoning.