Melt the butter in a medium-sized saucepan over medium heat, add the onion and cook, stirring, until the onion is translucent, about 8 minutes. Add the squash and the apple, stir, and cover it by 3-inches (7.5 cm) with filtered water. Add the sea salt and the cinnamon stick and bring to a boil over medium-high heat. Reduce the heat, cover and cook until the squash is tender through, about 25 minutes.