In a wide, deep sauté pan over medium heat, melt butter and add chopped onion. Sauté onion for 7-10 minutes, until lightly browned. Add the garlic during the last minute of cooking time, so it doesnt burn. Add chicken broth and bring to a boil. Add the greens, and boil gently, uncovered, stirring occasionally, for about 10 minutes. Greens will still be a fairly bright in color, but should be tender by this time.
Using a tongs or a slotted spoon, remove greens to a serving bowl, leaving the liquid in the pan, and set the greens aside. Simmer the liquid remaining in the pan for another 5 minutes or so, until it is reduced to 3 or so tablespoons. Season to taste with freshly ground pepper, and pour over greens in the serving bowl, stirring slightly to mix. Sprinkle with chopped bacon if using.
Note: It is difficult to fit all these greens in a standard 12-inch non-stick skillet, so if that is your largest pan, consider halving the recipe.
Also, you may substitute fresh pea tendrils for the chard, if you are lucky enough to find them available in season (spring).