In a large saucepan, warm the oil over medium heat. Add the cumin seeds. Once they start to dance in the oil, add the onion. Cook and stir for about 5 minutes. Add the garlic, pepper, turmeric, ginger, salt, and sweet potatoes and sauté for 3 minutes. Add the ground coriander and optional rasam powder and sauté for 1 or 2 minutes, until sweet potato is half cooked. Add 1 cup water and a bit more salt. Add the chard and continue cooking for 8 to 10 minutes, until the chard is also tender. Add the coconut milk, bring everything to a quick boil, and then reduce to a simmer, covered. Thin the stew with water if it seems too thick. Spoon the stew over rice, quinoa, or other cooked grain of your choice and garnish with the cilantro.