Scrub the sweet potato clean, then slice into thin rounds. Arrange the slices, overlapping, in a large roasting tin.
Bruise the lemongrass, then finely chop the tender parts. Peel and finely chop the ginger and garlic. Peel and finely slice the chilli.
Mix the sesame oil, stock, coconut milk and two-thirds each of the lemongrass, ginger and garlic. Pour over the potatoes, tuck in the lime leaves (or squeeze the juice on top and scatter over the zest) and sprinkle with the chilli.
Roast for 20 to 25 minutes, or until the potatoes are done. About 8 to 10 minutes before the cooking time is up, sit the fish on the potatoes and top with the rest of the lemongrass, ginger and garlic.
When the fish is just cooked, drizzle with soy sauce, if you like. Delicious served with green beans and mangetout.