Pre-heat oven to 350℉
Mix knotweed and strawberries together in medium bowl and sprinkle, sugar, cardamom and vanilla in, stirring to incorporate. Set aside until ready to use.
In a separate bowl, add oats, flour, ground ginger, and sugar, stirring to blend.
Cut butter into crumble topping, mashing in until all of the mixture is course and crumbly (don't overmix).
Spoon knotweed mixture into baking dish and drop crumble topping over by the spoonful, making to completely cover the fruit underneath.
Bake for 25 minutes, until fruit has softened and topping is golden brown and crisped.
If a little extra sugary sparkle is needed, sprinkle some demerara sugar over the top right before pulling from the oven.
Serve hot with a scoop of vanilla ice cream.