Crush biscuits to fine crumbs, either in a food processor, or by breaking them into a ziplock bag and crushing with a rolling pin. Either way, you’ll probably need to do this in 3 or 4 batches.
Mix in melted butter and stir until well combined.
Press the crumb mixture firmly into the bottom and sides of a 23cm springform pan, using either a small glass or the back of a spoon.
Place in the fridge while you prepare the next layer.
Mix all ingredients together until smooth and well blended. Pour into crumb crust and bake at 180 degrees Celsius for 20-25 minutes. Remove from oven, let stand 15 minutes while you prepare the topping.
Turn oven up to 230 degrees.
Stir all ingredients well, until sugar is dissolved. Spoon over filling. Sprinkle with nutmeg if desired.
Bake at 230 degrees for 10-15 minutes. Let cool at room temperature, then chill overnight.