Bring a large pot of salted water to a boil and cook bow tie pasta until al dente according to package instructions; drain and reserve ½ cup pasta water.
In a large skillet, melt butter over medium heat; add chopped onion and cook until soft and translucent, about 3 to 4 minutes.
Add minced garlic and cook for 30 seconds until fragrant.
Add ground beef to the skillet and cook, breaking it apart until browned and fully cooked; drain excess grease.
Season with salt, black pepper, paprika, and red pepper flakes if using; stir to combine.
Pour in diced tomatoes with green chilies if desired, simmer for 2 to 3 minutes.
Add milk or heavy cream and stir to begin forming the creamy sauce.
Lower heat and gradually add cubed Velveeta cheese, stirring until melted and sauce is smooth; add reserved pasta water if sauce is too thick.
Add cooked pasta to the cheese and beef mixture and gently stir to coat thoroughly.
Taste and adjust seasoning if needed.
Remove from heat, garnish with chopped fresh parsley, and serve immediately.