Heat a large, deep nonstick frying pan or a Dutch oven over medium heat. Once hot, add the black mustard seeds and cumin seeds. Stir occasionally and toss the seeds around for 1-2 minutes, or until they start to jump in the pan and smell very aromatic but not burning.
Add the coconut oil and once it’s melted, add the diced onions. Cook, stirring occasionally, until the onions are softened and start to turn golden, 5 to 7 minutes.
Add the garlic and ginger and cook for 1-2 minutes until very aromatic, stirring frequently to prevent burning and sticking.
Add the bay leaves and ground spices, and then add the chickpeas and stir them around to coat around in the spices. Allow them to cook for 5-6 minutes, turning only once after a few minutes, to allow them to get a bit browned and crispy.
Pour in the water, stirring around to scrape up any browned bits on the bottom of the pot. Pour in the crushed tomatoes, diced sweet potatoes, crushed tomatoes, and salt. Stir well and bring to a boil. Reduce the heat to medium-low and simmer, stirring occasionally, for 30 minutes, or until the sweet potatoes are tender and the stew has thickened.
Add the chard (or kale) and cover the pan. Cook for 5 more minutes until wilted. Remove bay leaves.
Stir in cilantro and lemon juice. Serve with fresh mint, sliced red onion, and coconut yogurt.