Cook the pasta according to the instruction on the package, drain and pour into a 9 x 13" baking dish.
Brown the ground beef in a skillet and drain; set aside.
While the beef is cooking, combine the Velveeta and milk in a saucepan and heat over medium low heat until a sauce forms. You can also heat the Velveeta and milk in the microwave, cooking in 20 second intervals, stirring in between, until a sauce forms.
Heat the skillet over medium heat and add the onions and garlic; cook until soft and starting to caramelize.
Add the black beans, green chilies and seasonings. Cook for 1 minute then add the tomato sauce and ground beef and mix together.
Add the Velveeta sauce and 2 ounces of the grated cheddar to the skillet and mix well.
Pour mixture over the pasta in the baking dish. Cover with remaining grated cheddar cheese and bake until hot and bubbly, about 20 minutes. Serve with sliced green onions and sour cream.