Melt butter in a large saucepan over medium heat; add onion and cook, stirring often until very soft, about 8 minutes. Add pumpkin, water, milk, syrup, salt and pepper; bring to a boil. Reduce heat and simmer for 10 minutes, whisking often. (Soup can be made 1 day ahead. Let cool and then cover and chill. Bring to a simmer before serving.)