¼cupunsalted butter (half stick, or 2 ounces)cut into cubes
½cupchopped pecans (optional)
Preheat oven to 375˚F.
In a medium sized bowl, mix together all the filling ingredients (rhubarb, sugar, flour, vanilla, cardamom). Turn the mixture out into a 2-quart baking dish.
Make the topping: put flour and salt in a bowl, add the butter cubes, and work the butter in with your fingers until the mixture is soft and crumbly. Add the brown sugar and nuts, if you're using them, and mix lightly with your hands.
Alternatively, pulse the flour, salt, and butter in a food processor until the pieces of butter are the size of peas, then mix in the brown sugar and nuts.
Scatter the topping over the rhubarb mixture, covering it in an even layer.
Bake at 375˚ for 40 minutes or so, until the filling is bubbly and the topping is lightly browned.