Combine the pears and nuts with most of the sugar (set aside about 2 tablespoons for the streusel) and let stand 30 minutes. Lots of juice will be released; that’s good.
Preheat the oven to 350 while that is going on.
Add the flour, cinnamon, and baking powder to the pears.
Beat the oil, vanilla, and egg together and add that, too.
Stir gently and pour into a greased 8 or 9″ round cake pan.
Combine the oatmeal, reserved sugar, and butter, then sprinkle the mix over top of the cake.
Bake 40-50 minutes or until it passes the toothpick test.