Preheat oven to 375 degrees F. In a 8 x 8-inch baking dish, mix together the rhubarb, brown sugar, white sugar, flour, balsamic vinegar, vanilla extract and ginger. Set aside while you make the crumble.

In a medium bowl, add the flour and cold butter. Using your hands, break up the butter until it resembles pea-sized bits. Add the roughly chopped walnuts, brown sugar, cardamom and salt. Place the crumble topping atop the filling; transfer to the oven and bake for 35 to 40 minutes, until the top is lightly golden brown and the filling is bubbly. Serve warm and I don't mean to be bossy but this crumble is really perfect with vanilla ice cream.