Add the onion and garlic to the skillet and cook over moderate heat until tender, about 4 minutes. Add the capellini and cook, stirring, until toasted, about 3 minutes. Add the thyme, butter beans, chicken stock, tomato puree and the reserved chorizo. Season with salt and pepper and bring to a simmer. Cover the skillet and simmer over moderate heat until the pasta is tender, about 2 minutes. Let the fideos stand for 1 minute, then spoon into bowls and serve.