Place the plum halves in a bowl and chop them (we just want to create an uneven and moisturized texture). Place the plum pieces in a non-stick pan over medium heat. Add the water, brown sugar and cinnamon and cook until soft (about 5 minutes).
10 plums, ¼ cup water, 2 tablespoons brown sugar, 1 tablespoon cinnamon
For the crumble, put the flour in a large bowl. Add the melted butter and run through it with your fingers until the mixture resembles breadcrumbs.
½ cup flour, ½ cup butter
Add the sugar, nuts and cinnamon and mix together.
½ cup brown sugar, ½ cup nuts, 1 tablespoon cinnamon
Transfer the plum mixture to a baking dish. Sprinkle the crumble mixture over it evenly and bake for 25-30 minutes (or until golden brown).