Prepare the sausage by removing the casing.
In a large saucepan, cover the beans with 3 inches of cold water and bring to a boil.
Cook over moderate heat until the beans are barely tender, about 30 minutes.
Heat the olive oil in a large saucepan.
Add the onion and bell pepper and cook over moderate heat until softened, about 7 minutes.
Add the sausage and cook for 4 minutes, crumbling it with a spoon.
Add the beans, tomatoes, stock, thyme and crushed red pepper and season with salt and pepper.
Cover partially and simmer over moderate heat until the beans are tender, about 20 minutes.
Add the potatoes, cover partially and cook until tender, about 15 minutes longer.
Stir in the parsley, if you like, and serve.