Like most baby food, this puree freezes well. Simply store leftovers in single-serving containers and freeze for up to two months. Defrost frozen baby food in the refrigerator overnight.
In a large saucepan, combine potatoes, onions, and enough cold water to cover by about an inch. Bring to a boil over medium high heat, then reduce to a simmer and cook until potatoes are very tender, about 10 minutes.
Add fish fillet to pot with potatoes and onions and continue to gently simmer until cooked through, about 5 minutes.
Using a slotted spoon, remove fish and vegetables from water and transfer to a food processor or blender. Add dill and pinch of salt. Puree, thinning out with reserved poaching liquid, as necessary to achieve desired consistency for baby.