A great buddy of mine is Bengali and a real lover of food. He would probably be the first to say that all Bengalis are. Once upon a time he lent me a cookbook written by his great aunt, Minakshie DasGupta , and it really is the authentic article when it comes to Bengali cuisine. I've asked him if he wants the book back but his response was such as to suggest that it is now on permanent loan. I'm OK with that.
The fact of the matter is that Minakshie DasGupta was the founder of what is purported to be Calcutta's most famous restaurant, Kewpies, now run by her children. You only have to look a little deeper into books by authoritative food-writers like Madhur Jaffrey  and Rick Stein  to appreciate the high esteem the restaurant and its cooks are held in.
Bengalis, given their geography, tend to eat freshwater fish and their recipes have presumably evolved around the mild flavour of them. When attempting to recreate their dishes I therefore lean toward our milder-flavoured sea fish - I'm not a huge fan of freshwater fish anyway. Dover sole, to my mind, has the ideal combination of flavour and texture to fit the bill, especially in the sense that it doesn't flake readily when in liquid. However, I would happily make this dish using skinned fillets of gurnard, John Dory and maybe even grey mullet. Conveniently, with the nets starting to go down, any or all of these should be available for the rest of the year.
Mustard oil features prominently in Bengali cuisine and I love cooking with it. Rick Stein  says that vegetable oil is no replacement, whereas Madhur Jaffrey  disagrees, and even suggests that olive oil is a substitute to consider. I, for one, have never been tempted to try that. Mustard oil is not that difficult to get hold of (see here, for example) but it is understandable to be deterred by the warning "for external use only" appearing on the label - I was compelled to investigate further. It is true that the oil is used as a treatment for hair (as is coconut oil in the south of India), but the explanation is made clear in a statement from a UK-based online Indian food supplier, Spices of India. In short, mustard oil contains too much erucic acid as to comply with EU Directive 80/891/EEC to be classified as a food. How helpful!
Having made this dish for years and years, and having consulted many different sources, it's difficult to attribute the recipe below to any single source. However, I would say that of anyone's, my version is closest to that in Madhur Jaffrey's "A Taste of India" , a favourite book.
You can make a big batch of the gravy for this dish and freeze it in portions. After it has been defrosted (the microwave is ideal) you can fry the fish and proceed as per the recipe. It makes for a very quick meal.
Ingredients (serves 1-2)
4 fillets of dover sole (or alternative fish), skinned and cut into chunky pieces
2 tsp mustard oil
1 tsp sunflower or vegetable oil
Garam masala, pinch (optional)
½ tsp turmeric, plus an extra pinch
1 tsp ground coriander
1 tsp cumin
1 tsp Kashmiri chilli powder, or a mix of chilli powder and unsmoked paprika, to taste
½ tsp salt, plus an extra pinch
1 tsp ginger puree
1 tsp garlic puree
¼ tsp nigella seeds
1 dried bay leaf
½ medium onion, finely chopped
3-4 dried red chillies, or to taste
3 green chillies, slit lengthways, or to taste
Coriander leaves, for garnish
Put the fish pieces in a non-metallic dish and sprinkle over some salt, a pinch of turmeric, a pinch of garam masala (if using) and a drizzle of lemon juice. Rub the fish well with the marinade and leave for 20-30 minutes,
In a small bowl mix the turmeric, coriander, cumin, chilli powder, salt, ginger puree and garlic puree with just enough water to make a paste.
Heat the mustard oil in a pan until it starts to smoke. Remove from the heat and, once the smoking has subsided return the pan to the heat and fry the fish pieces briefly on either side, then remove to a plate.
Add the sunflower or vegetable oil and once the oil has become hot add the nigella seeds, the dried red chillies and the bay leaf. When the seeds start to sizzle and the chillies and bay leaf have started to colour add the chopped onion. Reduce the heat to medium and fry the onions for a few minutes until they just start caramelising. Add the spice paste, fry for about 30 seconds stirring regularly, and then add 3-4 tbsp water to create a medium-thickness gravy and allow to simmer for a further 5 or so minutes, adding water as necessary.
Return the fish pieces to the gravy, increase the heat until the gravy just starts to boil and then let everything simmer for about 3 minutes until the fish is cooked.
Stir in a little lemon juice, garnish with coriander leaves, and serve straight away with boiled or steamed rice.
"Bangla Ranna, The Bengal Cookbook", Minakshie DasGupta (1982), pp. 99: https://www.amazon.co.uk/Bagla-Ranna-Minakshie-Das-Gupta/dp/8174762051
"A Taste of India", Madhur Jaffrey (1985), pp. 162: https://www.amazon.co.uk/TASTE-INDIA-Madhur-Jaffrey/dp/1862055467
"Flavours of India", Madhur Jaffrey (1995), pp. 152: https://www.amazon.co.uk/Flavours-India-Madhur-Jaffrey/dp/0563370777
Kewpies, Lonely Planet, accessed 20 August 2019: https://www.lonelyplanet.com/india/kolkata-calcutta/restaurants/kewpies/a/poi-eat/410776/356356
Mustard Oil, Spices of India, accessed 20 August 2019: https://www.spicesofindia.co.uk/cgi-bin/ss000001.pl?page=search&SS=mustard+oil&PR=-1&TB=O&ACTION.x=0&ACTION.y=0
"Statement regarding the use of Mustard Oil", Spices of India, accessed 20 August 2019: https://www.spicesofindia.co.uk/acatalog/Mustard-Oil-Statement.html