How to Make It
Soak the flageolets in 4 cups of water for 24 hours. Discard any beans that float; drain the beans.
In a medium saucepan, cook the onion and garlic in the oil over moderate heat, stirring, until softened, about 5 minutes. Add the tomato and cook just until it starts to break down, about 3 minutes. Add the Chicken Stock and beans and bring to a boil. Reduce the heat to moderately low, cover partially and cook until the beans are tender and the liquid is slightly thickened, about 1 1/2 hours; add water if the beans are dry. Season with salt and white pepper.
Add the potato and thyme and cook until the potato is tender, about 20 minutes. Stir in the butter and season with salt and white pepper.