We call this Green Gunge. Do NOT tell the kids what's in it - if you say 'fish' and 'spinach' they will gag before they even try. Just let them enjoy and you can smirk about how healthy it actually is.
1 fillet of fresh/frozen uncooked fish (it can by anything, cod, haddock, salmon, whatever)
3 potatoes, boiled and mashed
1/2 cup wilted fresh, or defrosted spinach
Handful grated cheese - whatever you have
Poach the fish in some milk in the microwave until just cooked (5 mins max).
Add the mashed potato, spinach and cheese and mix well.
Add some milk if it's too dry and taste - add seasoning or more cheese.
Blitz in the processor or mash by hand until it's all mixed up as a Green Gunge puree.
Serve as is, or with sausages, steak, burger pattie, chops, whatever.
My kids have been eating this since they were weaned and apart from adding ketchup and spicy sauce they still love it and they still don't know how healthy it is. Despite being VERY picky eaters they both happily eat fish and spinach now so my job is done.