Wrap the thyme, rosemary and bay leaf in a piece of cheesecloth and secure with kitchen twine to make a sachet. Heat the olive oil and butter in a large, heavy saucepan over medium heat. Add the onion, pancetta and garlic and saute until the onion is tender, about 3 minutes. Add the broth, beans, tomatoes and herb sachet. Cover and bring to a boil over medium-high heat, then decrease the heat to medium and simmer until the vegetables are very tender, about 10 minutes. Discard the sachet.