Roughly chop chard leaves and chop smaller stems into ¼" dice. You should be able to get about ⅓ cup diced chard stems, but it's OK if it's less.
In a small saucepan over medium heat, bring vegetable broth to a simmer.
Heat olive oil in a large saucepan over medium heat. Add onion, garlic, and the stems of the chard. Cook for about 5 minutes, stirring occasionally until softened.
Add one cup of the broth, cook and stir until almost absorbed. Now add the chard leaves, cannellini beans, and the rest of the broth. Cook for a couple minutes while stirring.
Add mascarpone cheese, lemon juice, thyme, salt, and pepper
Top with lemon zest before serving.