Gently wash the kale and beet greens under cool running water.
Remove and discard the ribs from the kale and chop.
Chop the beet greens.
Dice the onion.
Heat the olive oil in a large skillet on medium heat.
Add onion and sauté for 10-15 minutes, until glossy.
Add the beet and kale greens, salt, pepper and sauté.
Once the greens are tender, just before serving add the apple cider vinegar and red pepper flakes.