Preheat oven to 375 degrees. Line a rimmed baking sheet with parchment paper. Bake sweet potatoes on sheet until tender but not completely cooked through, 55 minutes to 1 hour and 10 minutes (depending on size). Let stand until sweet potatoes are cool enough to handle, about 15 minutes. Leave oven on.
Cut off the top quarter of each sweet potato; discard. Scoop out and reserve flesh, leaving a 1/2-inch-thick shell; set shells aside. Reserve half the flesh for another use. Coarsely chop the remaining flesh; set aside.
Heat oil in a large nonstick skillet over medium-high heat. Add onion, garlic, rosemary, salt, and red pepper flakes; cook, stirring occasionally, 3 minutes. Add kale; cook, stirring occasionally, until kale has wilted, about 5 minutes. Stir in reserved chopped sweet potatoes, the tofu, and 1/4 cup water. Cook until filling is heated through, about 45 seconds.
Place sweet potato shells on a rimmed baking sheet. Spoon filling into shells, dividing evenly. Cover with foil, and bake until heated through, about 30 minutes.