Wash and dice the bell peppers and onion. Give the spinach a rough chop. Then set the veggies to the side.
In a large bowl crack all of the eggs. Season the eggs with salt, pepper and add the half and half. Whisk the eggs until all ingredients are well combined.
In a large oven-safe cast iron skillet, melt the olive oil and butter over medium-high heat. Add peppers and onions and sauté for 3-5 minutes until translucent. Next add the chopped spinach and sauté for an additional minute. Season the vegetables with a little salt and pepper and make sure they are evenly spread out in the skillet.
Next gently pour the egg mixture over the sautéed vegetables. Then sprinkle the shredded sharp cheddar cheese over the entire skillet. Allow the egg mixture to cook on the stove for 2-3 minutes until the eggs start to barely set on the sides of the pan.
Next transfer the skillet to the preheated oven and bake for 20 minutes.
After 20 minutes, carefully remove the skillet and allow the frittata to rest. It may be puffy in the middle once it comes out of the oven, but it will flatten once it rests and starts to cool. Garnish with fresh parsley or sliced green onions and enjoy!
Notes
If you are using a 10-inch cast iron skillet then 8 large eggs works perfectly. If you are using a 12-inch skillet then you may want to use 10 eggs for a thicker, fluffier frittata.
I prefer white sharp cheddar cheese with this recipe, but if you can't find white cheddar then feel free to use regular sharp cheddar cheese.