Preheat the oven to 350ºF (180ºC).
To make the fruit filling, melt the 4 tablespoons of butter in a large saucepan and stir in the 1/4 cup brown sugar. Add the rhubarb and plums and a tablespoon of water.
Add the honey, cardamom (if using), cinnamon stick, and lemon peel, and cook for five minutes, stirring regularly.
Meanwhile make the topping. Mix together the dry ingredients and add the 14 tablespoons of melted butter, mixing quickly but lightly to form a crumbly texture.
Remove the broken cinnamon stick and lemon peel. Transfer the fruit mixture into a 2 quart (2l) low baking dish, then scatter the crispy topping on top. Do not press it down or it will get mushy. Scatter a tablespoon of granulated sugar on top and bake for 20 to 25 minutes, or until the top is golden brown and the juices are bubbling. (You may want to put a sheet of aluminum foil on the rack until the crisp in the oven to catch any bubbling juices.
Remove from the oven when the top is deep golden brown and you can plunge a paring knife into the middle and the knife meets no resistance, indicating the fruit is fully cooked.
Serve warm, with a bowl of thick cream and another bowl of muscovado sugar, for guests to add themselves.