How do I iron-fortify homemade baby cereal? I have been making baby cereal powder by grinding rolled oats, flax seed, and hemp seed into a powder. I prepare this cereal by adding water and heat.

Many commercial powdered baby cereals (and general breakfast cereals) are iron-fortified. 

What can I add to my homemade dry powder to fortify it with iron?

Note: Please hold off posting any opinions. I'm looking for factual answers with links to reference data.
Whole grain contains more iron than the more processed grain. So that might help. Seeds and nuts also contain a lot of iron. While nuts in themselves are a choking hazard for babies, ground nuts mixed with the rest of the stuff you grind should be no problem.