Heat the butter in a large heavy saute pan over medium-high heat. Add the butternut squash and season with salt and pepper, to taste. Cook until the squash is beginning to caramelize, about 6 to 8 minutes. Add the apples and continue to brown, another 5 minutes. Stir in 1 cup of water and the sugar. Cover, then reduce the heat to medium-low and cook until tender, about 15 minutes. Add the cinnamon and cook until the squash and apples are coated in a deep golden brown glaze, about 5 minutes more. Transfer the stew to a serving bowl and serve.