Place muesli, brown/coconut sugar, butter and coconut oil in a food processor and blitz until very fine and crumbly and the mixture holds together when pinched between your fingers.
Press two thirds of the mixture into lined tin. Bake for 12-15 minutes until golden. Remove from oven and allow to cool while you make the filling.
To make caramel filling, place butter and golden syrup in a pot and stir over low-medium heat until the butter has melted. Turn off the heat and stir in condensed milk until smooth.
Pour caramel all over the base and gently smooth out, then dot with remaining crumble mixture and sprinkle with a little extra muesli. Return to oven to bake for a further 20-25 minutes or until crumble topping is golden and caramel is bubbling and darkening a bit at the edges. Remove from oven and leave to cool before placing in the freezer or fridge to set and cutting into pieces (it’s up to you how far it goes, but because its so decadent I reckon you can get 30 or so pieces out of it).