Brown beef in a skillet; drain. Add salt, pepper, and chili powder to taste.
Add beef, cheddar cheese, and onion to each tortilla and roll up. Place side by side in a baking dish.
To make sauce: Melt butter, slowly add flour, and stir to make a paste. Add milk slowly, stirring constantly.
Add Velveeta and green chilies. Cook until melted.
Pour sauce over enchiladas and bake at 350°F for 20 minutes.
