Remove any large stems from the greens and discard. Tear the leaves into bite-sized pieces.
In a deep pot or wok, sauté the onion in a tablespoon or two of vegetable broth until mostly faded to pink, about 4 minutes. Add the chopped garlic and red pepper and another tablespoon of broth and cook, stirring, for another minute. Add the mustard greens, 2 tablespoons of broth, and cook, stirring, until greens are wilted but still bright green, about 3-5 minutes. Stir in the salt, if using. Remove greens and onions from pan with a slotted spoon and place in a serving dish, leaving any liquid in pan.
Add the balsamic vinegar, soy sauce, and agave or sugar to the liquid in the pan (if there is no liquid, add 2 tablespoons of broth). Add the chickpeas and cook, stirring, over medium heat until the liquid is reduced by about half. Spoon the chickpeas over the greens and drizzle the sauce over all.
Serve warm, with additional balsamic vinegar at the table.
Reduce the sodium by using lite soy sauce and unsalted chickpeas.