You’ll want to start with a reasonably sized, closable vessel for the oven (a dutch oven may be too big in this case) and a couple of thick cuts of fish (fillets are probably too thin, steaks are cut the wrong way, I used cod loin, but if that is not cost effective in your area, or is out of season, make the smallest possible adjustment from this option you can).
In addition, you want a mango (soft, but not mushy), a large stick of lemongrass, hot peppers (I used habanero I oven dried two Summers ago), olive oil and basic seasonings.
Apply a thin (but complete) layer of olive oil into your cooking vessel. Lay the fish flat, keeping the upper surface dry of the oil. Cut the lemongrass into a handful of large pieces which you bruise with the back of a knife, but keep whole (you need to remove them later, and you really don’t want pieces of lemongrass ending up in your creamy sauce), and scatter these and the hot peppers around (but not on top of) the fish.
Season the top of the fish.
Slice the mango into long, thin strips and arrange these on top of the fish.
Cover and put into a 225f degree oven for at least an hour, two might be better.
Transfer the fish to plates, and get out the lemongrass (compost it). Everything else goes into a blender or food processor to be rendered smooth and creamy.
Dress the fish and serve with saffron rice. Depending on how many hot peppers you used, have alcohol or dairy on hand.
If you have extra sauce left over, it makes a fantastic sauce for pasta with lump crab meat.
Well, unless you don’t like sweet heat.