Butter a deep baking dish. We used a 3-quart CorningWare oval casserole.
Prepare the fruit. Put the sliced plums in a medium bowl, sprinkle on the brown sugar and flour, then the Chambord, and mix it all together with your hands until it creates a smooth, thick sauce. Pour the fruit into the baking dish.
Prepare the topping. Put all the topping ingredients into another bowl and mix together with a pastry fork or your hands. Blend together to make a uniform coarse meal.
Scatter the crumb topping all over the fruit. You may not need it all—save any leftovers in the fridge because you likely are going to find a use for it very soon, such as making this recipe yet again.
Preheat the oven to 350ºF. Put the baking dish in the oven and set a timer for 40 minutes. It is done when the fruit is bubbling away briskly around the edges and the topping is turning golden. Let it sit until it is not blazing hot. It is lovely when it’s warm, it is lovely at room temperature and, to be honest, it is also lovely ice-cold, straight out of the fridge, on a suddenly hot day or when you just cannot be bothered to pop this in the toaster oven for a quick reheat.