Steaming is a time-honored way of cooking fishes, mostly because the results are so good. Microwaves aren't exactly time-honored, but they seem to be a great way to steam fishes without ...well... steaming them.
The exotic Bengali word for steam is bhapa, and the most famous of the bhapa varieties are undoubtedly the one where mustard is involved. When wrapped in banana leaf, it's also called paturi. This is not the only way that bongs know to bring fish and mustard together, or even fish and steam - but it is the most well known.
Bhapa fish is a really simple dish; just two things are absolutely required - mustard paste and fish. And, of course, a wrapper of some kind to steam in - traditionally banana leaf, but I use parchment when that's not around. You coat the fish with the mustard paste and steam it for enough time; this yields a fairly interesting (if not wonderful) mustard fish. In India, it's made with hilsa - the holy grail of steamed fishes - or bhetki. Here, in the land of the hilsa-deprived, the fishes that works best are flaky, high-fat fishes such as halibut, cod, salmon or chilean sea bass. Shad, a common hilsa substitute, does not have enough fat. Salmon steak is easy to get and has a strong-enough taste to stand up to lots of mustard and tastes great though somewhat non-traditional (salmon isn't available in Bengal).
The best mustard paste is made using Colman's English Mustard Powder(only the powder, mind you, NOT the paste). However, this mustard, in the refined British way that the refined British are, is a smooth uniform yellow paste - very unexciting. You want your mustard a little flecked, a little stubbly, so to get a bit of texture I usually add freshly powdered black mustard (rai) seeds. They don't taste as strong as the Colman's, but look a whole lot more real. Make the paste dry, with just a tiny amount of water.
Now for the fancy touches. First, a thickener that makes the sauce cling to the fish. Once choice is posto (poppy-seed) paste. Don't go overboard - about a third the volume of mustard should be enough posto. Be light, posto doesn't have much flavour so too much of it will give you a very dull fish.
The other option (the one favoured by bong grandmothers) is fresh coconut ground to a fine paste. Add as much of it as you like; tastes great. No fresh coconut? Use a touch of canned coconut cream, though I recommend heating is slowly before to thicken it a little (and slow is important, or you will end up with coconut oil). I like coconut cream, so I add it anyway, even when there's fresh coconut. Also, you have to cool it before adding it to anything, or you'll end up cooking the fish with the sauce well before any steam can be let off.
Finally, the toppings - green chillies (deseeded) and kalonji (nigella seeds) crackled in oil. A few drops of raw mustard oil adds an extra kick. Mix up the paste, coat the fishes generously, then take a deep breath. Put a sheet of parchment (banana leaf, if you're intent on being fancy) on the table, place the coated fishes on the paper, wrap it in, fold and staple the sides closed (yes, staple - the kind you get at Staples). Then, put it into a microwave, remember to press the start button and wait. It's about usually three minutes per medium steak - about 250g (a little over 1/2 lb) of fish, but you'll have to experiment with your particular microwave and fish; luckily, it's quite tolerant and wont overcook easily.