Preheat the oven to 350° F. Shred the cheddar cheese, chop the kale and onion, set aside.
Over medium heat, warm up a skillet and add the butter. Once the butter has melted, add the chopped onion and sauté until soft and translucent. Add the cooked onions to a 9" deep dish pie pan.
In a large bowl, add the eggs, shredded cheddar cheese, heavy cream, and salt. Use a whisk and mix until well combined. Fold in the chopped kale to the egg mixture. Pour the egg mixture over the onions in the pie pan.
Bake in a preheated 350° F oven for 30 minutes or until the quiche has set in the middle. Allow to cool for about 10-15 minutes before slicing. Serve & enjoy!