Step 1

Preheat oven to 375° F.

Step 2

Slice the onion into thin strips and saute in a 10″ cast iron skillet along with 1 Tablespoon of oil and 1 Tablespoon of butter.  Season generously with salt as it will help the onion sweat and caramelize.  Allow to saute for 35-40 minutes until very tender, stirring frequently until a deep brown color is achieved.  Set aside 1/4 cup of the onions.

Step 3

Add the sliced mushrooms and cook for an additional 5 minutes or until all the liquid has been released and evaporated.

Step 4

Add the chopped spinach to the skillet and cook for just a minute until spinach is tender.

Step 5

In large bowl, crack the eggs and add the cream or milk.  Add 1 teaspoon of salt, 1/2 teaspoon pepper, paprika, thyme and oregano and red pepper flakes (if desired).  Whisk well until eggs are frothy.   Add the shredded cheese and stir to combine.  Pour the egg mixture into cast iron skillet and top the frittata with the reserved onions.

Step 6

Carefully put the frittata in the preheated oven on the middle rack and bake for 22-25 minutes until middle is set but not overcooked.  The frittata will continue to cook in the skillet when you take it out of the oven so do not overcook.   Allow to cool 10 minutes before serving.

Step 7

Top with sour cream, avocado or chives.