First, gently boil the can of condensed milk in a large lidded pan of water. This turns the condensed milk into caramel. It will take about 2 hours, boiling gently. Keep topping up the water to make sure the can is covered at all times. Remove and allow to cool before opening. For the crumble topping, mix together all the ingredients apart from the oats until you get what resembles fine breadcrumbs, then add the oats. Using your hands squeeze the mixture to form a ball then break it into small pieces again. This will ensure your crumble topping has texture once cooked.
Peel the apples, remove their core and cut into 2cm chunks. Heat a heavy based saucepan on the stove, melt the butter then add the apples, sugar and cinnamon. Cook gently for 5-6 minutes till the apples start to soften slightly. Remove from the heat and allow to continue to cook in their own heat for 3-4 minutes then chill. Once chilled spoon a third of the apples into the serving dish and top with half of the caramel. Repeat this process finishing with the final layer of apples.
Top with the crumble mix and place into the oven at 170C for 10-15 minutes until the crumble is golden brown and cooked through.
Serve with your favourite custard.