Boil the potatoes for about 10 minutes, or until soft.
Meanwhile, heat up a pot with some oil and fry the onion and garlic until softened. Add about two tablespoons of garam masala, a tablespoon of curry powder, and cayenne pepper and salt to taste. Add approximately 1/3 of the can of crushed tomatoes to the spices and simmer while the potatoes are finishing. (These are measurements are approximate, as I sort of made this up as I went along. Feel free to add more of the curry and garam masala if you don't think it's spicy enough!)
Drain potatoes and add them, along with the chickpeas, to the tomato mixture. (You can mash the potatoes up first if you like.) Add water as necessary to achieve a tomato gravy consistency.
Allow this to simmer together for a while to allow the spices to marinate more fully, say 20 minutes. Add water as necessary to maintain the consistency.