In a cast iron skillet, cook onions in olive oil over medium-high heat until they begin to brown. Stir in brown sugar, and continue cooking for a few minutes.
When onions are brown and tender, stir in chard and olives. Cook until chard is slightly wilted. Stir in capers and salt, and continue cooking until chard is completely wilted, about 3 minutes. Season with black pepper and squeeze lemon over the top.