Heat the oil in a pot over medium-high heat. Add onions and garlic cook 2 to 3 minutes, or until softened. Add the cumin and chipotle peppers and cook 1 minute more. Pour in the beer, bring to a simmer and reduce the beer by half. Add the stock, lime juice and brown sugar and bring to a bowl. Add the mussels and cover and cook until the mussels just open (discard any that do not). Divide mussels among heated bowls, spoon over the broth, sprinkle with chopped cilantro and serve
Note: Chipotle peppers are available in the Mexican food aisle. Store the unused peppers in a tight-sealed jar in the fridge.