Add the potatoes to a large pot and cover with cold water by 2 inches. Bring to a simmer and cook until soft, about 16 to 20 minutes. Drain and return to the pot; cook over medium-low heat for 1-2 minutes more to evaporate the moisture. Mash with a potato masher and fold in the butter, heavy cream, cream cheese, and sour cream; season to taste and set aside.
Meanwhile, prepare the sauce. Heat the butter in a small skillet and stir in the flour; cook to make a roux, stirring for about 5-10 minutes. Turn off the heat.
Heat the olive oil in a separate skillet over medium-high heat. Add the garlic and cook until golden. Add the onions and sauté until translucent. Stir in the thyme and A-1 sauce and heat through. Turn off the heat and pour in brandy, then turn the burners to high and flambé until the alcohol is cooked out. Stir in the mushrooms and cook until their natural water releases. Whisk in the heavy cream, broth and the roux and simmer on low until reduced to your desired consistency. Turn off the heat and whisk in the sour cream. Season to taste.
Preheat the oven to 350°F and heat a grill pan on high heat. Season the steaks with salt and pepper to taste and grill for about 2 to 3 minutes per side, or until grill marks appear. Transfer to the oven and roast until the internal temperature reaches the desired doneness (120°F for medium-rare and 130°F for medium).
Sauté the spinach in a large skillet over medium heat until just wilted. Sprinkle with salt and pepper
Place each steak on a serving plate and add the mashed potatoes and spinach. Spoon the sauce over the steaks and serve.