Preheat your oven to 350. On a rimmed baking sheet, drizzle the olive oil over the potatoes and scatter salt over the top. Rub the salt and oil all over the potatoes until they’re completely coated. Bake until they give little resistance when poked with a fork or knife, about an hour depending on your oven and the size of your potatoes.
While the potatoes are baking, make the filling. Over medium heat, add the ground beef to a skillet and cook until fully brown, breaking the meat apart as it cooks. Once the meat is browned, add the carrot, onion, celery, and garlic, and cook until the vegetables are mostly softened, about 5 minutes.
Add the flour and stir to coat the meat and vegetables. Any fat in the skillet should be fully absorbed by the flour. If it isn’t, remove the excess with a paper towel. Holding the paper towel with tongs, blot at the excess fat. The paper towel will absorb it. Add the tomato paste, stock, and Worcestershire sauce. Stir until the tomato paste has dissolved, and cook until the filling has thickened, about 3-5 minutes. Once the filling has thickened, taste and add salt and pepper if needed. Remove from heat and set aside.
Once the potatoes are done, let them cool for 5-10 minutes, or until you can safely handle them without burning yourself. Cut them in half and scoop out the insides using a spoon, leaving 1/4” of potato on the bottom and sides. Put the scooped-out potato bits into a medium bowl.
To the medium bowl with the scooped-out potato bits, add the shredded cheddar, butter, and milk. Blend with a hand mixer or mash with a potato masher, just until everything is blended together and there are very few to no lumps. Taste and add salt if needed. Don’t worry if the cheese hasn’t melted — it’ll melt in the next step!
Assemble the potatoes. Preheat the oven to 400. Fill the potato skins with the meat mixture and spread the mashed potatoes over the top. If you’d like, drag a fork over the top of the mashed potatoes to add grooves. This will make them look good! Once the potatoes are assembled, bake at 400 for 10-15 minutes, or until the tops are golden with some browning at the edges.