Maybe commercial cereal has a taste in them that's why your baby will take it better?
You may wish to cook porridge with fish stock and add in some vegetable/fruit puree.
For me, I will cook the porridge with some fish stock, and add in a cube of frozen vegetable/fruit puree.
Not sure how old your child is. Mine is 6.5months and currently I give her carrot, apple, sweet potato, pumpkin purees.