1. Mix flour, brown sugar and margarine until crumbly. Generously grease 8-cup ring dish; coat with 1/4 cup crumbly mixture. Reserve 3/4 cup remaining mixture for topping.
2. Stir sour cream, egg, orange peel, baking soda and cinnamon into remaining crumbly mixture just until moistened. Spread evenly in ring mold. Sprinkle with reserved crumbly mixture and the nuts. Microwave uncovered on high (100%) 3 minutes; rotate ring dish 1/2 turn. Microwave until wooden pick inserted near center comes out clean, 3 to 5 minutes longer. Let cool 10 minutes on heatproof surface (do not use rack). Invert on plate; cover with serving plate and invert again.