Peel the potatoes and cut into 2.5cm/1in cubes. Place in a pan of lightly salted water and cook for 10-15 minutes until just tender.
Meanwhile, wash the spinach under cold running water, place in a large pan, with only the water which is clinging to the leaves, cover and cook for 3-4 minutes over a medium heat, shaking the pan from time to time. Drain well and squeeze out as much of the water as you can. Set aside.
Slice one of the onions. Heat the oil in a large pan, add the onions and sauté until golden brown. Add the cumin and coriander seeds and cook for 1 minute.
Chop the remaining onion and place in a food processor or liquidiser together fried onions. Stir well and cook over a low heat for 5 minutes.
Add the reserved potatoes, spinach, chilli powder, coriander powder, salt and fenugreek to the onions and cook over a low heat for
Stir in the tomatoes, cover and simmer for 10 minutes. Serve immediately.