Cut squash in half lengthwise and remove seeds. Whisk together vinegar, maple syrup, and 2 Tbsp oil. Brush the flesh side of squash halves maple-oil and sprinkle with 1/4 tsp salt and dash of pepper. Put squash flesh side down in baking dish, brush skin side with maple oil mixture and sprinkle with 1/4 tsp salt and dash of pepper. Roast in oven until squash is fork tender, 30-40 minutes. Allow to cool, then scoop enough flesh out to leave a 1/4 in. border of flesh all around. Leave scooped out flesh in relatively large chunks.
Heat 1 Tbsp oil in a medium saucepan over medium-high heat. Add onions and cook, stirring occasionally, until soft and golden brown. Add the rice, curry powder, cinnamon, cayenne and 1/2 tsp salt, and stir until the spices are toasted, about 1 minute. Add 2 cups water and bring to a simmer covered, stirring occasionally, until the rice is tender and most of the liquid is absorbed, 30-40 minutes (add more water if needed). Remove from heat and add to the chunks of butternut squash along with the remaining maple-oil, cherries, parsley, sage, 1/2 teaspoon salt and a few grinds of black pepper.
Evenly stuff the scooped-out squash halves with the filling, then drizzle with the remaining 1 tablespoon oil and bake, uncovered, until the filling is warmed through, about 30 minutes. Cut each in half crosswise and transfer to a serving platter. Sprinkle with walnuts and parsley. Serve warm.