Slice up onion into half moon shape -- cut it in half and then slice it legnthwise.
Preheat a skillet and melt the butter, add in onion slices and sauté until lightly caramelized. They will become a light golden brown color.
Meanwhile, in a large bowl, whisk together the eggs, heavy cream, and salt.
Shred the entire block of cheddar cheese and fold into the egg mixture.
Next, you’ll add the onions to a standard pie pan. If you don't have a pie pan handy, a cast iron skillet will work also.
Pour the egg and cheese mixture right over the onions.
Bake for about 35 minutes or until the quiche has set. It will give a light golden brown color to the top because of the cheese.
Allow to cool for about 15-20 minutes before slicing up.
Notes
Add in idea: if you want to give this recipe more of a boost, bacon is a great option! Cook 6 slices of bacon, cool and crumble. Stir into the egg mixture before pouring into the pie pan.