Heat oven to 170c and line a slice tin (28cm x 18cm) with baking paper. In a large bowl add the oats, flour, brown sugar, salt and baking powder, and mix together well. Pour in the melted butter and stir until all of the dry ingredients are damp. The crumble mixture should be the texture of wet sand and hold together in a clump when you squeeze it.
Transfer ⅔ of the mixture to the baking tin and spread out to the edges. Use the back of a spoon to push the mix down to form a firm, even base. Bake in the oven for 10 minutes, then remove and allow to cool a little.
Dollop the jam and peanut butter over the slice base then swirl together. Scatter the raspberries over the top (if using), then sprinkle the rest of the slice mix evenly over the filling. Bake in the oven for another 25 minutes or until the top is just starting to turn golden.
Remove from the oven and leave to cool on a wire rack. When the slice is cool, transfer it to the fridge and let it set for a few hours before slicing into generous squares.