Preheat oven to 375 degrees Fahrenheit (190 degrees Celsius). In the bowl of a mixer, combine the crust ingredients until the mixture resembles coarse crumbs. Press into a greased 9×13-inch baking dish. (This can also be done in an 8×12-inch pan for higher bars or a 10×15-inch for lower ones.) Bake for 15 minutes.

Meanwhile, prepare filling. Beat eggs and sugar until thick and lemon colored, about five minutes. Stir in flour, almonds, orange zest, and baking powder.

When crust is finished baking, arrange plums in an even layer over it. Pour custard mixture over plums.

Combine sugar, flour, and oil for topping and sprinkle over filling. Bake for 30 minutes. Remove from oven and sprinkle with sliced almonds. Return to oven and bake an additional 10–15 minutes.