Combine ground beef and onion in a large skillet. Cook completely and drain. Add rice and seasonings to the skillet. Stir in water. Bring water just to a boil and simmer just until absorbed, about 5 minutes.
Fill each tortilla with beef and rice mixture. Roll up and place in a 13×9 casserole dish.
In a small saucepan, melt butter or margarine over medium heat. Whisk in flour and cook for 2-3 minutes, stirring frequently. Whisk in milk, bring to a boil and cook until thickened. Remove from heat and stir in cheese. Continue stirring until cheese is completely melted. Stir in sour cream.
Pour sauce over tortillas and either cover and refrigerate or bake right away. Cook at 350 degrees for 20 minutes, until heat through and sauce is bubbly.
