Stir together dry ingredients for streusel topping, then use pastry blender, fork or two knifes to cut in the vegan butter until the mixture is rough crumbs. Refrigerate until needed.
Wash, peel and slice apples, set aside. Preheat oven to 350F.
In small bowl, stir together ground flax and water. Set aside to let thicken. Prep a 9 inch springform pan with oil or spray. Stir together flour, baking powder, cinnamon and salt.
Whisk together oil and sugar until well blended and almost fluffly. Add flax eggs and combine.
Add in flour mixture, alternating with milk. Stir or fold gently and only until the ingredients are just incorporated. Spread into prepared pan. Top with apple slices, then streusel topping.
Bake for 50-70 minutes, or until toothpick inserted near the center comes out clean. Allow to cool, serve with vegan ice cream or whipped cream if desired.