Preheat oven to 425°. Heat a large skillet over high heat. Cook beef until no longer pink, about 5 minutes. Scoop out beef and set aside.
Reduce heat to medium; add onion and carrots. Cook, stirring occasionally, until softened, about 5 minutes. Stir in tomato paste. Add flour; cook, stirring, 2 minutes.
Add Worcestershire sauce, 1/2 cup water, and beef. Season with a pinch each of salt and pepper. Simmer until thickened, stirring occasionally, about 5 minutes.
Meanwhile, make potatoes: pierce each potato a few times with a fork and microwave until tender, about 8 minutes. When they are cool enough to handle, cut an oval in the top of each potato and scoop out the insides, going as close as possible to the skin without breaking it.
Mash the potato insides with milk and butter, and season to taste with salt and pepper.
Place the potatoes on a foil-lined baking pan. Stuff the skins with the beef mixture, packing it in tight to use all of it. Top with mashed potato mixture.
Bake until tops are just starting to brown, around 20 minutes. Serve!