Bring a large pot of salted water to a boil. Cook tortellini according to package directions until tender. Reserve ½ cup of pasta water, then drain and set aside.
Pat steak pieces dry and season with Cajun seasoning, smoked paprika, garlic powder, salt, and black pepper. Heat olive oil in a large skillet over medium-high heat. Sear steak bites for 2-3 minutes per side until browned and cooked to desired doneness. Remove from skillet and set aside.
In the same skillet, reduce heat to medium. Melt butter, then add garlic and cook until fragrant (about 1 minute). Stir in red pepper flakes, Dijon mustard, and lemon juice. Add parsley and chives, mixing well.
Add the tortellini and steak bites back into the skillet. Toss until coated in the cowboy butter sauce. Add reserved pasta water as needed to create a silky sauce. Stir in Parmesan cheese.
Plate hot and garnish with extra Parmesan, parsley, and a sprinkle of Cajun seasoning for color and spice.
