Dhal is simply the Indian word for lentils. The best kind to use for this are the red split lentils which most supermarkets stock.
This recipe is taken from Delia Smith's Frugal Food
8 oz (225 g) red lentils
2 level teaspoons ground ginger
1 level teaspoon ground cumin
1 level teaspoon turmeric
2 potatoes, peeled and diced
2 tablespoons butter
1 large onion, peeled and sliced
1 small green pepper, de-seeded and chopped
1 level teaspoon Madras curry powder
1 clove garlic, crushed
4 tomatoes, peeled and chopped
You will also need a medium saucepan and a medium heavy-based frying pan.
In a saucepan containing 1½ pints (850 ml) of water add one teaspoon each of ginger, cumin, turmeric and salt, then bring it all to the boil.
Stir in the lentils, let it come back to a gentle simmer and cook for 5 minutes.
After that add the diced potato and continue cooking until the lentils have turned mushy (making sure the mixture doesn't stick to the bottom of the pan by giving it a stir now and again).
While that's cooking, heat up the butter in another pan and fry the onion and pepper in it (over a fairly high heat) until the onion has browned – then lower the heat and stir in the Madras curry powder (if you like your curries hotter, you can add more), the garlic, the remaining ground ginger and the chopped tomatoes.
Cook for a minute before adding the lentil mixture, then taste, season with more salt and cook gently for a further 5-10 minutes (stirring from time to time).