Place all the ingredients in a mixing bowl.
Use the dough hook of your mixer or your hands to knead the mixture into streusel.
Cover the bowl and place in the fridge.
In a separate bowl, whisk together the flour, baking powder, salt, and sugar.
Cut the butter into pieces. Add to the dry ingredients along with the beaten egg and knead into a smooth dough.
Cover the bowl with plastic cling film and place in the refrigerator for 30 minutes.
Wash the apples and pat dry. Peel, core, and cut into quarters. Slice each apple quarter thinly lengthwise.
In a large skillet, add the remaining 2 tablespoons sugar, and the 1/3 cup water. Bring the mixture to a boil, stirring frequently.
Add the apple slices to the mixture. Gently stir to coat the apple slices.
Cover the skillet, lower the heat, and steam the apples for 8-10 minutes.
Use a slotted spoon to transfer the apple slices to a colander. Drain and set aside.
Preheat the oven to 350 degrees F. Grease and flour an 8-inch spring form pan.
Remove the crust dough from the refrigerator Press the cake dough into the pan, making sure the dough goes up the sides roughly 2 inches. The dough will be sticky. Wetting your hands will make this easier. You want a frame deep enough to hold the apple slices and the streusel.
Gently transfer the steamed sugared apple slices to the prepared crust. Evenly distribute the sage streusel over the top.
Bake for 30 minutes. Remove the cake from the oven and let cool for 10 minutes.
Run a knife around the inside of the pan and pull the ring off.
Gently run the knife under the cake and carefully slide it off the pan bottom onto a serving plate.
Slice and serve warm (with vanilla ice cream if you prefer)