Heat the oil in a large skillet over medium heat. Add the onion and saute until soft, about 5 minutes. Add garlic, ginger, curry powder, and cumin and continue to cook for about 1 minute until garlic is cooked but not burnt. Add the tomatoes, water, salt, and pepper; bring to a boil, cover, and lower to a simmer. Cook until the potatoes are tender, about 20 minutes; stir occasionally. After the 20 minutes, add zucchini and cashews. Cook, uncovered, until the zucchini is tender, about 10 minutes. Mix in the lemon juice. Serve over rice with a spoonful of yogurt and some chopped cashews on top.