PREHEAT oven to 350°.

GREASE bottom and 1/2" up the sides of a 9x5x3" loaf pan; SET aside

In a large mixing bowl, COMBINE flour, baking powder, baking soda, cinnamon, salt, nutmeg, and ginger; MAKE a well in the middle of the mixture; SET aside.

In a medium mixing bowl, COMBINE eggs, mashed bananas, sugar, and oil.

POUR egg mixture all at once into flour mixture; STIR just until moistened (batter should be lumpy).

FOLD in walnuts; SPOON batter into prepared pan; SET aside.

In a small bowl, COMBINE brown sugar and flour; using a pastry blender (or a fork), CUT in butter until mixture resembles coarse crumbs; STIR in walnuts.

SPRINKLE topping evenly over batter.

BAKE until a wooden pick inserted in the middle comes out clean, about 55 to 60 minutes; NOTE: to prevent overbrowning, cover loosely with foil the last 15 minutes.

COOL in pan on wire rack for 10 minutes; REMOVE from pan; COOL completely on wire rack.

WRAP; STORE overnight in refrigerator before slicing.