Preheat oven to 350 degrees.
In a large, heavy skillet or pot, brown onions in moderately hot oil along with the cumin seeds and mustard seeds. Add the potato pieces, peeled or not, as you choose, and stir to coat each piece with spices and continue to stir for several minutes.
Thin tomato paste with about 2/3 cup water. Stir into the pot. Add vegetables, one at a time, cooking for aminute or so between each addition, and put in cooked chickpeas last. If your pot is not oven proof, transfer mixture to one that is. Cover with a lid or seal with foil, and bake for abuot 45 minutes, checking after the first 20 minutes.
The consistency of this curry should be thick, rather than watery, but add more liquid if necessary to prevent burning. Stir occasionally to keep ingredients from sticking to the bottom of the pot. Serve over rice of with Indian bread.