Preheat the oven: preheat the oven to 180°C / 350-375°F / Gas Mark 4-5.
Apples: use a vegetable peeler and cutting board to peel each of the apples. Then use a small knife to remove the seeds and chop the apples into small pieces. Pour the water, lemon juice and brown sugar into a saucepan and add the apples. Turn the temperature to high and cook for 4-5 minutes or until the liquids and sugar has been absorbed. You will need to stir regularly throughout this time. Once cooked, allow to cool.
Baking dish: lightly grease a square (24 x 24 cm or 9.4 inch) baking dish with cooking oil then line it with 2 overlapping sheets of baking paper with a 2-3 inch overhang on each side.
Sugar and butter: pour both the brown sugar and white sugar into a large mixing bowl. Then place the butter into a microwave safe jug and heat in the microwave using the high setting in 30 second increments until melted (usually less than 1 minute). Use a tea towel to grab the jug out of the microwave and set it on a heatproof surface. Stir the melted butter then pour it into the bowl and combine with the sugars.
Eggs and vanilla: next add the 2 eggs and vanilla to the mixing bowl and stir till the batter is consistently combined.
Flour and apple pieces: next add the plain flour to the bowl, stir to combine. Then add the cooked apple pieces and use a spatula to gently fold in.
Baking dish: use the spatula to transfer the blondie batter into the prepared baking dish. Then spread this out so that it reaches all 4 corners and is in a reasonably even layer.
Topping: add the butter for the topping to a heatproof jug and microwave on high for 30 seconds to melt. Then pour the butter over the top of the blondies. Next sprinkle the combined sugar over the top. Once done place the baking dish into the preheated oven on the middle shelf. Let the blondies bake for 35 minutes.
To serve: use oven mitts to remove the baking dish from the oven when the blondies are cooked. Then let them cool in the dish for at least 20 minutes. Remove them by pulling the paper up and out of the tray and rest them on a wire rack. Let them cool for another 20 minutes. Then slice into 16 even portions and serve warm with vanilla ice-cream.