Wash kale well in a sink full of cold water. Keep swishing until all the sand/soil has fallen off. Change water and rinse again. Cut kale into strips about 1/2-inch wide. Shake off water.
Heat oil in large skillet over medium heat. Saute onion for 2 minutes, then start adding kale in small batches and stir until it all cooks down and you can cover the skillet. Cover, lower heat, and steam until done to your liking. We like ours tender-crisp so this takes less than 10 minutes. Stir in red pepper flakes and salt and pepper to taste. Serve with balsamic vinegar if desired.