melt butter and sugar in a saucepan, stir in the oats.
simmer the blueberries with the zest of lemon, and rest of lemon cut in quarters, until juice just beginning to run. take off heat, remove lemon after squeezing it all out into the bluberries, add cassis.
roll out marzipan.
In deep dish, spread half the oats, half the blueberry mix, top with layer of marzipan, then cover that with rest of blueberries, and rest fo oats. Bake 20 mins, at 155C.
when cool, top with whipped cream.