Blend the spices together and set aside. Peel and slice the onions and potatoes into rounds 1/4 inch thick. Combine the yogurt, chili, garlic, ginger, and spice mix. In a large bowl, toss the sliced onions and potatoes. Add the yogurt mixture and spread the vegetables in an even layer in a shallow baking dish. Pour the stock over the top. Bake in preheated 350 degree oven for 1 1/2 hours, until thoroughtly cooked and browned on top