This is what I found on wikipedia ....So I guess a nage could be made with anything as long as it served this certain purpose
Miso, tomato, and coconut milk sounds stange and delicious...sounds like a pretty creative variation...maybe he was vegan or decided to make it like that for some religious reason.
You could saute chopped onions and herbs with rice bran oil, then add the tomatoes...and maybe some sake(what kind of sake would be best used for this...Junmai daiginjo? or just the junmai...or futsu?) Whatever type of fish...maybe sole...or tuna...salmon? poach the fish then add coconut cream(and maybe a little bit of cornstarch?)
That could possibly be extremely delicious...you should try it.
A nage is a flavoured liquid used for poaching delicate foods, typically seafood. A traditional nage is a broth flavoured with white wine, vegetables, and herbs, in which seafood is poached. The liquid is then reduced and thickened with cream and/or butter.[1]
[edit]Background
Cooking something à la nage translates as “while swimming” (French nage) and refers to cooking in a well-flavoured court-bouillon.[2] Eventually the term "nage" itself came to refer to a broth which, while light, is strong enough to be served as a light sauce with the dish itself [3], unlike a court-bouillon which is not. Ingredients such as tomatoes may be added.[1]