Preheat the oven to 200C and place 12 tulip muffin cases in a 12 hole tin.
Start by making your crumble mixture: sift the flour into a bowl and add the butter, cubed. Use your fingertips to rub the two together until they resemble craggy breadcrumbs. You want big and small chunks. Add the oats, sugar and ground cinnamon and mix gently.
Make your custard as per the instructions of the back of your tub or packet. Usually this involves combining the custard powder and sugar with a little milk, whisking to form a smooth paste. Then add the remaining milk, whisk again and microwave for 6 minutes at 600W, stirring halfway. Leave your custard to stand for a few minutes and to cool a little before filling a piping bag with it.
Brown your butter in a saucepan over a low-medium heat until it starts to bubble gently. Keep heating until it turns a beautiful golden colour and little flecks begin to appear, then remove it from the heat and leave to cool.
Sieve your flour, baking powder, bicarbonate of soda and ground cinnamon into a bowl.
In a separate bowl, beat the eggs and sugar until pale and creamy. Add the browned butter and whisk again.
Pour the wet ingredients into the dry ingredients and use a rubber spatula to gently mix everything together until well combined.
Add your apples or pears (or seasonal fruit) and mix again until combined.
Snip the end of your piping bag with a pair of scissors and insert the tip of the bag into each muffin. Squeeze a little custard into each muffin until it starts to swell. Repeat for all 12 muffins.
Sprinkle over your prepared crumble mixture, coating the top of each muffin generously and evenly.
Use a portioner to divide the mixture between your 12 muffin cases, then turn your oven down to 180C and bake them for 35-40 minutes until risen, golden brown and a skewer inserted into the centre comes out clean.
Leave to stand for 10 minutes before removing from the tin and eating warm from the oven.