1 large onion
1 bunch kale (about 2 pounds)
2 teaspoons vinegar (or to taste)
¼ teaspoon crushed red pepper flakes (or more to taste)
salt and ground black pepper to taste
Sauté the onion in a large, non-stick skillet on low heat, until translucent. While the onion sautés, thoroughly rinse the kale. Remove and discard the large stem ends, and coarsely chop the leaves.
Add the moist kale to the onions and cook, covered, stirring occasionally, until the leaves are wilted but still bright green. Stir in the vinegar and red pepper flakes. Add salt and pepper to taste, and serve immediately or at room temperature.