Soak the dal for the spice paste in a bowl of water for 20 minutes. Drain well and set aside.
To make the spice paste, put the fenugreek seeds in a kadhai or small, heavy-based dry frying pan (skillet) over medium heat and stir-fry for 1-2 minutes, or until roasted.
Remove from the pan and transfer to a small blender. Add all the remaining ingredients, including the soaked dal and 4 tablespoons water, and process to make a smooth paste.
Bring 375 ml / 13fl oz (1 1/2 cups) water to a boil in a large, heavy-based pan. Add the potatoes and turmeric and season with salt. Cover, reduce the heat to medium, and simmer for about 10-12 minutes, or until the potatoes are tender.
Stir in the spice paste and simmer for 1-2 minutes. Stir in the yoghurt and heat through, stirring constantly to prevent curdling for a further 1-2 minutes, or until hot.