Tip:To adapt a slow-cooker recipe to a conventional oven, follow these guidelines: add more liquid, to accommodate for greater evaporation; bring the dish to a boil over high heat in a Dutch oven, then cover the pot and put in a 350°F oven. Plan on the dish taking roughly half the time to cook in the oven as it would in the slow cooker.
Place two layers of paper towels on a plate, then top with the tofu, then another two layers of paper towels, and a large plate. Leave for about 15 minutes to drain. Cut the tofu into 1-inch cubes. In a small bowl, stir together the brown sugar and curry paste. Add one-third of this mixture to a zip-top, gallon-size plastic bag. Add the tofu and gently coat it with the marinade, using your hands to massage it. Close the bag and refrigerate.
Put the sweet potatoes into the slow cooker, then mix with another third of the sugar-curry mixture and all of the salt and pepper. Heat 2 Tbs. of oil in a 10-inch, heavy sauté pan over medium-high heat. When hot, add the ginger, lemongrass, and garlic, and sauté until softened, about 2 minutes. Add the final third of the sugar-curry mixture, plus the flour (and the remaining 1 Tbs. of oil if the pan is very dry), and cook, stirring, until no white flour is visible, no more than a minute. Add the coconut milk, pumpkin, lime juice, and stock, and bring to a boil over high heat, whisking vigorously to remove any lumps. Cook until smooth, thickened, and some bubbles appear, about 4 minutes. Pour over the sweet potatoes.
Cover the slow cooker and cook on low until the sweet potatoes are very tender but still retain their shape, about 4 hours. Stir in the chard and marinated tofu, cover, and cook on high until the chard is tender, about another 20 minutes. Serve immediately.