Preheat the oven to 180 C. In an 8-inch round baking dish (or use a square dish. The size and shape of the dish isn't very crucial as long as it fits the apples with enough room for the crumble), combine the apple chunks, cinnamon, ginger and sugar. Mix well and set aside to help the apples release their juices. You can cut the apples into smaller pieces to help them cook faster and become softer. I like them with a little bite!
Now make the topping. In a bowl, combine the flour, cinnamon, brown sugar and salt (if using). Mix well, then add the cubed butter.
Rub the butter into the flour with your fingertips till you have a crumbly mixture, resembling coarse breadcrumbs. Toss in the biscuits and walnuts. Spread this mixture all over the apples evenly, making sure the apple pieces are covered so they don't dry out in the oven. If the butter seems to be softening or the mixture seems sticky, chill it in the fridge for 15 minutes before adding it to the apples because cold butter makes for a better crumble.
Bake for 20 to 25 minutes until the top has started browning. Cover with foil and continue to bake for another 20 minutes or until the apples feel tender when poked with a fork. If you feel the top isn't browning fast enough, you can increase the temperature to 200 C for a few minutes and bake without the foil.
Allow the crumble to cool, uncovered, for 10 minutes, then serve with vanilla ice cream! Tastes best eaten fresh as the crumble topping will soften as it sits, especially once covered. However, you can store it in the fridge for 3 to 4 days, warming it up before eating. Happy baking!