Bring vegetable broth to bare simmer in saucepan over medium heat and set aside.
Meanwhile, chop smaller chard stems into 1/3-inch dice and slice leaves into thin strips.
Heat olive oil in large saucepan over medium heat, add onion, garlic and chard stems. Cook for about 5 minutes, stirring occasionally, until softened. Add one cup of the broth, cook and stir, until almost absorbed. Add chard leaves, cannellini beans and the rest of the broth. Cook and stir for a couple of minutes, add mascarpone cheese, lemon juice and thyme. Season with salt, pepper and more thyme if needed. Top with some grated lemon zest before serving.