Preheat oven to 180°C/350°F/Gas Mark 6
Line a 20cm round or a 20cm by 20cm square tin with baking parchment leaving an overlap to lift them out with.
In a large bowl, combine melted butter and sugar.
Add flour and mix, then add oats. It will be thick.
Press mixture into the base of the tin, creating a raised lip around the edges.
Bake for 10 to 15 minutes until slighty browned.
Spread the coconut in an even layer over a large baking sheet and bake for 8-10 minutes. Stop to stir every two minutes.
While crust is baking, combine sugar, cornflour, tumeric and salt in a saucepan.
Add coconut milk, water, lemon juice and zest. Cook over medium heat while whisking until bubbling. Reduce heat and cook for 10 minutes until thick.
Pour into the oat base and sprinkle with the toasted coconut.
Let set for at least 2 hours in fridge before cutting into slices or squares.