In a large pot or Dutch oven cook the bacon over medium-high heat until crispy. Remove bacon from the pot and drain on a paper towel lined plate, set aside to cool. Wipe or pour out all but 1 tablespoon of the bacon fat.
Lower the heat to medium and add the carrots and onions to the pot. Season with 1 teaspoon of kosher salt and ½ teaspoon of ground black pepper. Stir often, and cook until carrots begin to soften and onions start to brown, about 5-7 minutes.
Add the Marsala wine to the pot and increase heat to high. Simmer for 8-10 minutes, or until wine reduces by half.
Add the chicken stock to the Marsala and carrot-onion mixture in the pot and simmer for about 2-3 minutes.
Add the heavy cream, rotisserie chicken, bullion or chicken base, fresh thyme, peas, kosher salt, and black pepper. Stir until well combined and reduce heat to medium-low and simmer for about 5 minutes.
In a small bowl, combine softened butter and flour into a paste. Spoon ¼ of the sauce into the butter-flour paste and stir until combined. Add mixture to the pot and stir continuously until well combined. Simmer for 2-3 minutes, or until mixture begins to thicken and coats the back of a wooden spoon. Season to taste with salt and pepper.
Preheat broiler to 450 degrees or on high.
Spray a large casserole dish with nonstick cooking spray. Pour the stew mixture into prepared casserole dish. Using a rubber spatula, scoop ½ cup of mashed potatoes onto the stew mixture. Repeat until top of stew is completely covered by the mashed potatoes.
OPTIONAL: Using a fork, draw it across the mashed potatoes making a pattern to create ridges. Repeat until pattern covers the entire top of the potatoes.
Place casserole dish under the broiler for 3-5 minutes, or until potatoes begin to brown. Allow the casserole to cool for a minute or two before serving.