Combine first 4 ingredients in a small bowl; cut in margarine with a pastry blender until mixture resembles coarse meal. Stir in basil. Combine milk, sour cream, and onion, stirring well. Add to dry ingredients, stirring with a fork just until dry ingredients are moistened.
Sprinkle 1 teaspoon flour evenly over work surface. Turn dough out onto floured surface, and knead 4 or 5 times. Roll dough to 1/2-inch thickness; cut into rounds with a 2 1/2-inch biscuit cutter. Place rounds on an ungreased baking sheet. Bake at 425° for 13 to 15 minutes or until golden.