In a medium bowl, or in the bowl of a food processor, combine the flour, brown sugar, salt, cinnamon, and cardamom. Mix/pulse to combine.
Cut the butter into small cubes then add to the flour mixture. If mixing my hand use a pastry blender or fork to cut in the butter. If using a food processor, processes until the mixture forms coarse crumbs.
Stir in the chopped nuts and set aside.
Preheat oven to 350°F. Grease a 9-inch springform pan with baker's spray (like Pam with Flour). Line the pan with parchment paper and spray again, then use a pastry brush to spread it evenly.
Peel, core and dice the apples. Place into a medium bowl and toss with the lemon juice to keep them from browning as you prep the cake batter.
Sift the all-purpose flour, baking powder, baking soda, and cardamom into a mixing bowl. Add the salt, then whisk well to combine, set aside.
In the bowl of a stand mixer combine the oil, brown sugar, granulated sugar and vanilla. Mix well to combine.
Add the eggs to the oil/sugar mixture and mix until pale and a little fluffy (about 1 minute)
Add the flour mixture to the wet ingredients in two batches, alternating with the buttermilk. Mix well (about 30 seconds) after each addition.
Toss the diced apples with 2 tablespoons of flour then fold into the batter. Pour the batter into the prepared springform pan. Top evenly with all of the streusel mixture and press down gently so that it sticks to the cake batter well.
Bake at 350°F for 45-55 minutes or until a toothpick comes out clean. Allow to cool on a wire rack for at least 10 minutes before unmolding from the springform pan.
I used almonds for the topping, but you could easily use pecans or walnuts (or whatever other nuts you have on hand). I don't always have buttermilk so I will substitute it using regular milk and vinegar. Just put 1 Tablespoon of vinegar in a measuring cup, then fill to the 1 cup mark with regular milk. Stir and let sit for a few minutes before using in the batter.
Make it Dairy Free: You can replace the buttermilk with non-dairy milk and 1 Tablespoon of vinegar as described above. Replace the butter in the topping with margarine or another non-dairy butter replacement (like Earth Balance).