Goldfish are onmivores, like most Carp species, so there is a huge variety of foods available - variety is the spice of life with our fishy friends too! I'm not sure about steaming vegetables beforehand - maybe the idea is to make them more easily digestible, but I'm sure a lot of the goodness will be lost in the process - I always feed veg raw to my omnivorous/vegetarian fish personally. Most of the "vegetable" part of a Carp's diet in the wild will consist of algae rasped from rocks or aquatic plants, so something like spinach might be the nearest approximation - it certainly contains a lot of beneficial nutrients, so is definitely worth offering! The bulk of most small to medium sized fish's diets consists of small invetebrates, so things like Daphnia, Cyclops, Mysis, Bloodworm, Black mosquito larvae and even small live crickets can substantially enrich your Goldfish's diet, which will in turn enrich its quality of life - never a bad thing!
Have you tried goldfishpellets "not pond pellets though as they clog the filter" you can also feed them frozen peas either slightly blanched or defrosted and chopped small, apart from the fact that the fish love them (in moderation of coarse) the peas help as a laxative to help clear out any constipated fish or fancys with swimbladder trouble, as for steaming the veg I think you will find its blanching, so as not to cook all the goodness out of it.