Bring a large pot of water on to boil. Heat the oil in a large skillet over moderately low heat. Add the onion and cook until softened. Add the red pepper, celery, and garlic and cook for 3 minutes more. Stir in the white beans and chicken stock. Bring to a boil and simmer, mashing the beans slightly with the back of a spoon for 5 minutes. Add the hot pepper flakes and tuna, season with salt and pepper, to taste, and simmer until heated through. Keep warm while you cook the pasta.
Salt the boiling water and add the pasta. Cook until al dente following the instructions on the back of the package. Drain and transfer to a bowl. Toss with the sauce and serve garnishing each portion with some of the celery leaves.