Put oil in large pot over medium heat; when oil is hot, add onion and cook, stirring occasionally until soft, about 5 minutes.
Add beans with their liquid, tomatoes, bulgur, chili and rosemary and sprinkle with salt and pepper. Cook, over medium-low heat, until tomatoes have broken up and flavors begin to blend, about 12 minutes, adding water to make it as soupy as you like.
Add arugula and continue to cook, just until it wilts. Taste and adjust the seasoning, and serve.