Preheat capsicums on a baking tray and grill, using tongs to turn them as each side blackens. Place the blackened capsicums in a paper bag, close tightly and allow them to cool for 20 to 30 minutes. Peel the skin off the capsicums then discard the stem and all the seeds. Dice the flesh and set aside.
In a large saucepan over medium heat, cook the onion and garlic in the oil for 5 minutes, or until onion is soft. Add the capsicums and cook for 2 to 3 minutes.
Add the chicken stock and the cannellini beans, stirring well. Blend or puree in a processor and return to the saucepan over low heat for 5 minutes.