Cook the beef and onions: In a large pot or Dutch oven, heat the oil over medium heat. Add the ground beef and onions, cooking until the beef is browned and the onions softened, about 8 minutes. Drain excess fat if needed.
Add vegetables and water: Stir in the diced potatoes and tomatoes. Pour in the water and bring to a boil. Reduce the heat and simmer for 20–25 minutes, until the potatoes are tender.
Season and add beans: Stir in the chili powder, salt, and pepper. Add the beans and continue simmering for another 15–20 minutes to meld the flavors.
Thicken the chili: In a small bowl, whisk the flour with a few spoonfuls of hot broth until smooth. Stir the slurry back into the pot and cook for 5 more minutes, until slightly thickened.
Serve hot: Ladle into bowls and serve with crackers, cornbread, or biscuits. Garnish with cheese, sour cream, and chives if desired.