Butter a deep, springform 23cm round cake tin and line with baking parchment, then preheat your oven to 180C/350F.
Peel and quarter your apples, removing the core and thinly slice.
Place the 25g of butter into a large saucepan and heat gently to melt. Add in your apple slices and the cinnamon, then pop a lid on. Leave to simmer for 5-10mins on a low heat, stirring occasionally until the apples have softened slightly. N.B. You can check this by sticking a knife through them. In the meantime, make your cake: combine the flour, sugar, baking powder and salt in a bowl. Rub in the butter using yours hands until the mixture looks like fine breadcrumbs.
Whisk together your eggs and 4tbsp of milk, then add this to the breadcrumb-like mixture and combine thoroughly until a batter/dough forms. Spoon into your tin and level the surface with a spatula.
Your apples should be a little softer by now so remove them from the heat and evenly layer onto your cake. I like to concertina them from the outside-in, spiralling and layering until you have no more apple slices left.
To make the streusel topping, combine the flour, sugars, salt, oats, cinnamon and nutmeg in a bowl. Rub in the butter using your fingertips again to make a course crumble mixture. Scatter this on top of your apples.
Bake for 50-70mins until a skewer inserted in the center comes out clean. Keep an eye on it – overcooking will make it dry, so I suggest checking it after 50mins and if your skewer isn't yet clean, monitor it every 5 mins thereafter.
Leave to cool in the tin before unwrapping the springform tin. Serve with cream, custard or vanilla ice cream.