Dissolve the butter in an enamelled saucepan, then add the kale, after thoroughly washing and cutting it into two-inch pieces; place the saucepan over a gentle heat, shaking it frequently. Peel and slice the potato and onion, and place them, together with the salt, water and sugar, with the kale. Boil one hour, strain, return to the saucepan, add milk and sago, replace over the fire and stir for ten minutes. Strain again into a tureen, and serve with sippets of toast.