Preheat the oven to 375 degrees.
Heat 1 tablespoon of butter in a large cast iron skillet over medium-high heat. Add the mushrooms and onion then cook, stirring occasionally, for 5-7 minutes, or until the mushrooms are golden brown and the onion is softened.
Add the minced garlic and cook, stirring constantly, for 1 minute. Season with sea salt and freshly cracked pepper, to taste. Remove the mushroom mixture from the skillet onto a plate; set aside.
Heat the remaining 3 tablespoons of butter in the same cast iron skillet over medium-high heat.
Add the flour and cook, whisking constantly, for 3 minutes.
Slowly add the chicken broth and milk while constantly whisking.
Add the sour cream and cream cheese then simmer, stirring often, for 7-10 minutes, or until the cream cheese is melted and the sauce has thickened.
Add the chicken, mushroom mixture, and poppy seeds to the sauce and stir until combined. Taste and season with a bit of sea salt and lots of cracked pepper, to taste.
Make the topping by combining the crushed sleeve of Ritz crackers and the melted butter together in a bowl; mix until well combined.
Sprinkle the top of the casserole evenly with the parmesan cheese followed by the crushed Ritz topping.
Place into the oven and bake for 15-20 minutes, or until casserole is bubbling and the topping is golden brown.
Remove from the oven and allow to cool for a few minutes. Top with fresh parsley and serve. Enjoy.