Tip the rhubarb into a saucepan with the sugar and Port, if using. Cover and simmer on a very low heat for 15 mins, adding more sugar if you want. When soft (but still holding its shape) and sweet enough, pour the rhubarb into a medium baking dish.
Heat the oven to 200C/180C fan/gas 6. To make the topping, rub the flour and butter together with your fingers until you have a soft, crumbly topping. Now add the sugar and nuts, mixing together with your hands.
Scatter the topping over the rhubarb and bake for 30 mins or until golden brown on top. Serve piping hot with thick vanilla custard.