Place a large enameled cast iron dutch oven on the stove. Add a tiny splash of grapeseed oil and warm over medium high heat. Add the diced bacon and cook, stirring occasionally, until the bacon bits are nicely browned and crispy. Turn off the heat and use a slotted spoon to remove the bacon bits to a paper towel lined plate. Keep the drippings in the dutch oven.
Season the beef liberally with kosher salt and ground black pepper. Put the dutch oven back on the stove over high heat. When the bacon fat is shimmering, add the beef and brown well on all sides, turning the beef when it releases easily in the pan and develops a dark brown crust. Brown well on all sides. Don't skimp on this step. You are building the flavor here!
When the beef is browned on all sides, remove it from the pan and set aside on a plate. Reduce the heat to medium and add the onion, carrot, and celery to the dutch oven. Saute until softened, about 8 minutes.
When the vegetables are soft, add the garlic, thyme, rosemary, and bay leaf and stir for 1 minute, until fragrant.
Add the dry sherry, Worcestershire sauce, tomato paste, and flour and stir to combine. Let cook for a minute or two to thicken slightly. Next add the wine beef broth, water, reserved beef, and bacon to the dutch oven. Stir to combine. Bring to a boil, then reduce to a simmer. Cover the dutch oven with the lid, but keep it just slightly ajar. Simmer for 2 and a half hours, stirring every 30 minutes or so, or until the beef is tender and easy to shred with a fork.
When there is an hour left on the stew, start working on the mashed potato topping.
Bring a large pot of water to a boil and add the peeled potatoes. Boil for about 20 minutes, or until the potatoes are easily pierced with a knife. Drain the potatoes and return to the empty pot. Add the butter and 1/2 cup of the half and half. Use a potato masher to mash until smooth. Add 1/4 cup more of the half and half if the potatoes are dry, then season liberally with kosher salt and ground black pepper to taste. Set aside to cool slightly while you wait for the rest of the stew to finish cooking.
When the stew is done cooking and the beef is tender, turn the heat off on the stove and preheat the oven to 400 degrees Fahrenheit. Use two forks to shred the beef directly in the dutch oven. Add in the frozen peas and stir to combine.
Carefully scoop the mashed potatoes over the top of the stew, spreading evenly with a spatula or back of a spoon. Be sure to spread them all the way to the edges of the dutch oven. Creating a good seal on the edges with the mashed potatoes will help prevent the stew from bubbling over.
Finally, sprinkle the shredded Mozzarella and cheddar cheeses over the top of the mashed potatoes. When the oven is preheated, put the dutch oven inside the oven and bake, uncovered, for 15 minutes or so, until the mashed potatoes are warmed and the cheese topping is melted.
Remove from the oven and let rest for at least 15 minute before serving. When ready to serve, scoop the stew with the mashed potato topping into individual serving bowls and enjoy!