Cut the onions in half. Then cut the ends off of each half and peel the onions. Slice the onions into thin half moons and separate the rings with your fingers.
Preheat a cast iron skillet or griddle over medium high heat (approximately 350-400 degrees F).
Place the onions directly on the griddle or the cast iron skillet and place the avocado oil and butter on top. Sauté the onions for about 10-15 minutes stirring them every few minutes until the the onions are soft and a light brown color.
Then they are ready to serve with your favorite type of burger or steak!