Toss steak evenly with 1 tbsp avocado oil and 2 tbsp Cajun seasoning.
In a large pan over medium heat, add steak bites and cook undisturbed for 2 minutes to develop a crust. Flip and cook another 2 minutes.
Reduce heat to low, add 2 tbsp butter and 2 cloves garlic, toss and cook 1–2 minutes more.
Remove steak from the pan and tent with foil to keep warm.
In the same pan over medium-low heat, add remaining 2 tbsp butter and 3 cloves garlic. Cook until fragrant, about 30 seconds.
Add beef stock and increase heat to medium. Simmer 5–7 minutes until slightly reduced.
Meanwhile, cook tortellini according to package instructions. Drain.
To the sauce, stir in Cajun seasoning and black pepper.
Reduce heat to low, then add heavy cream and stir to combine.
Add cubed Velveeta and whisk until fully melted and smooth.
Simmer about 5 minutes until slightly thickened, then stir in parmesan until melted.
Add tortellini to the sauce and stir to coat. Bring to a low boil for 1–2 minutes to thicken.
Remove from heat and top with steak bites before serving.