Heat oil in a large (5- to 7-qt.) Dutch oven or other heavy pot over medium-high. Add onion, stirring often, until softened and starting to brown, 6 minutes. Stir in sweet potatoes, water, beans, tomatoes, chili powder, pepper, and salt. Bring to a boil over high heat, stirring occasionally.
Reduce heat to medium-low and simmer until potatoes are tender and chili is thickened, about 20 minutes. Spoon into bowls and add toppings, if using.
