Melt the butter in a saucepan. Add the chopped onion and cook gently until soft but not brown. Add the flour and stir, cooking for 1 minute. Gradually add the first measure of milk while stirring. Increase the heat and stir continuously until the sauce thickens and boils. Add Wattie's frozen Super-greens and continue cooking for a further 2 minutes. Remove from heat. Fold in the smoked fish. Season to taste and set aside to cool.