Prep the frozen Spinach by thawing it first, then pressing or squeezing all the water out using a clean tea towel, cheesecloth, or a fine mesh strainer.
Heat a 12" Cast Iron Skillet over medium-low heat.
Add the diced onions and butter, sautéing until the onions are just turning translucent but haven't really softened quite yet.
Add the Flour, Kosher Salt, and White Pepper to the onion and butter mixture, stirring to coat the onions, and cook for about 2 minutes.
Begin slowly pouring in the half and half while constantly stirring. Once the sauce has begun to thicken and bubble, add the Cream Cheese and Sour Cream. Slowly stir the mixture to incorporate the ingredients thoroughly.
Fold in the freshly grated Parmesan Cheese until a thick sauce has formed.
Lastly, add the chopped Spinach stirring to combine, and then let the creamed spinach cook and meld for about 5 minutes.