Soak the beans in plenty of water for about 12 hours and drain them. Peel and wash the onion then mince it finely. Clean the celery, eliminating all the filaments (keep the more tender leaves aside) and slice it in small pieces.Pop the beans and the celery in a saucepan, add the bay leaf and, once peeled and lightly squashed, the garlic; cover the mix with abundant water, bring to a boil and let it cook for 80 minutes, adding the salt only halfway through cooking. In the meantime wash the chards and the celery leaves kept aside in cold water; drain and cut them in strips.Drain the beans, eliminate the clove of garlic and the bay leaf, put them in the vegetable mill, keeping a spoon full aside, obtaining a creamy and smooth purée; add 2 tbsp of water and stir. Pour the purée in a pan and heat, keep stirring with a wooden spoon; only at the end add the olive oil and a pinch of salt and ground pepper.Bring to a boil abundant salted water and cook the pasta; 2 minutes before the cooking point, add the chards and the celery leaves. Drip the vegetables and the pasta al dente, mix with the bean purée, decorate with the whole cannellini beans kept aside and serve.