Peel and coarsely grate the potatoes. Place in a colander or large sieve. Trim the leek. Halve it and rinse out any grit. Finely slice it. Add to the potatoes with 1 tsp salt. Scrunch together. Put a small plate in the colander and weigh it down with a couple of tins so it's squashing the veg. Put the colander in the sink and leave to drain for 30 mins.
Peel and finely chop the red onion. Tip into a bowl. Slice the tops and bottoms off the oranges so they sit upright on the board, then slice off the skin and white pith. Slice the segments out of the membrane. Add to the onion.
Halve the chilli. Scrape out the seeds and white bits if you prefer a milder flavour. Finely chop the coriander stalks (keep the leaves for later). Add them both to the bowl with 1 tsp cumin.
Halve the avocado. Scoop out the stone, then use a spoon to scoop the flesh out of the skins. Finely chop it and add to the bowl with a pinch of salt and pepper. Stir to mix. Put to one side.
Press down on the plate in the colander to squeeze out any last moisture from the potatoes. Tip onto a plate lined with kitchen paper and briefly pat dry with kitchen paper.
Beat the egg in a large bowl. Add the potatoes and leek and stir together to mix well. Add the flour with and pinch of black pepper. Stir well. Turn your oven on to its lowest setting.
Warm a large frying pan for 1 min, then add 1/2 tbsp olive oil. Drop 4 tbsp of the fritter mix into the pan to make 4 fritters. Fry over a medium heat for 4-5 mins till golden brown underneath. Flip and fry for another 4-5 mins. Slide onto a plate.
Repeat with the rest of the fritter mix, adding a little more oil each time, till you have approximately 12 fritters. Arrange them on 2 warm plates. Spoon over the avocado salsa. Crumble over half the pack of feta. Garnish with coriander leaves and serve with a handful of watercress.