I have made this recipe many, many times! I play around with ingredients and whatnot- and it still seems to be pretty foolproof. My husband gives his stamp of approval too!
A few changes I often do:
-I never have tarragon on hand- usually I'll use a bit of dried dill instead (dill = seafood, right?)
-I nix the clove of garlic. This is mostly because raw and raw-ish garlic makes my mouth and lips tingle- I think I might have a really low-key allergy to it or something. Maybe I'll have to try garlic powder sometime
-I use haddock and cod interchangeably. And, if it's like a filet vs. a square (not sure what official word would actually be), I tuck the really thin part under to make it more square-ish. But- if you really like this topping (which you're probably going to) I could see one wanting to cover as much surface area of fish as possible.