Put a pan of salted water on to boil. Scrub and roughly chop the potatoes. When the water is boiling, add the potatoes to the pan. Simmer for 8-10 mins till they are just soft, then drain well.
While the potatoes simmer, prepare the veg. Peel and finely chop the onion. Trim the celery and finely slice it. Halve the pepper and scoop out the seeds. Roughly chop it. Peel and crush the garlic.
Warm a deep frying pan or wok over a medium heat for 1 min. Add 1 tsp each fennel and coriander seeds. Toast for 30 secs, then add the onion and celery with 2 tbsp water. Stir in 1 tsp each smoked paprika and ground cumin. Add a pinch of two of cayenne pepper (it's spicy, so add as much or as little as you like). Season with salt and pepper.
Cook and stir the onions for 5 mins, till the onions have softened a little. If they start to stick, add a splash more water. Scoop the onions to the edge of the pan onto a plate.
Put the pan back on the heat and add 1 tbsp oil. Add the potatoes and the pepper. Stir and fry for 8-10 mins till they're golden and a little browned in places.
Add the onions back to the pan. Add the garlic. Stir to mix everything together. Pour in 250ml boiling water. Lay the hake fillets in the pan. Cover and cook over a medium heat for 10 mins till the hake is white, opaque and flakes easily when pressed with a fork.
Serve the hake and spicy potatoes with a handful of lamb's lettuce.