1. Preheat oven to 350 degrees F. Grease bottom and 1/2 inch up the sides of one 9x5x3-inch or two 7-1/2x3-1/2x2-inch loaf pans; set aside. In a large bowl combine flour, baking powder, baking soda, salt, cinnamon, and nutmeg. Make a well in center of flour mixture; set aside.
2. In a medium bowl combine eggs, banana, sugar, and oil. Add egg mixture all at once to flour mixture. Stir just until moistened (batter should be lumpy). Fold in nuts. Spoon batter into prepared pan(s). If desired, sprinkle Streusel Topping over batter.
3. Bake for 55 to 60 minutes for 9x5x3-inch pan or 40 to 45 minutes for 7-1/2x3-1/2x2-inch pans or until a wooden toothpick inserted near center comes out clean (if necessary, cover loosely with foil the last 15 minutes of baking to prevent overbrowning). Cool in pan on a wire rack for 10 minutes. Remove from pan. Cool completely on a wire rack. Wrap and store overnight before slicing.
In a small bowl combine brown sugar and flour. Using a pastry blender, cut in butter until mixture resembles coarse crumbs. Stir in walnuts.
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