Add the onion, stir to coat with oil, then saute for 10-15 minutes, stirring occasionally. This slow cooking over low heat sweetens the onions. You know they're ready when you taste one.....no pungent onion flavor, just luscious sweetness! This can take longer if your onions are thicker.
Add the kale on top along w/ 1-2 tablespoons water.
Increase the heat to medium and cover.
Steam the kale for about 5-10 minutes until the kale is wilted and tender. Remove the lid and stir to incorporate the onions.
Add some salt to taste. Sprinkle in the vinegar and pepper flakes and saute another minute or two to evaporate any leftover moisture.
Taste and adjust seasonings. Try not to eat the whole batch all to yourself!