Heat oil in heavy large skillet over medium-high heat. Add onions and sauté until tender and translucent but not brown, about 10 minutes. (Can be prepared 2 hours ahead. Let stand at room temperature. Rewarm over medium heat before continuing.).
Add lime juice to onions and simmer until liquid is slightly reduced, about 3 minutes. Season to taste with salt and pepper. Remove onions from heat. Stir in cilantro leaves.