Heat the butter in a large pan. Add the onion, garlic, potato, celery, and bell pepper and mix well. Heat the vegetables in the butter until sizzling, then turn the heat down to low. Cover the pan and "sweat" the vegetables gently for 10 minutes.
Pour the stock, season to taste and bring to a boil. Lower the heat, replace the lid and simmer for about 15 minutes.
Stir in the milk, butter beans, and corn, then add the sage. Simmer for 5 minutes. Serve.