Wash and dry the potatoes and pierce them a few times with a fork. Place the potatoes onto a sheet pan and drizzle with olive oil. Rub the oil over the potatoes to coat and season with salt. Bake the potatoes for 1 hour or until cooked through and they can be easily poked through to the centre with a knife. Set aside to cool slightly until cool enough to handle.
Meanwhile, place a large skillet over medium heat and melt in 2 tablespoons of butter. Add in the onion and season with salt and pepper. Cook, stirring frequently, until the onion has softened and is lightly golden brown, about 5 to 6 minutes. Stir in the garlic and cook for another 30 seconds. Turn the heat up to medium-high, add the ground beef and cook, stirring frequently, until browned and cooked through, about 7 to 8 minutes. Turn the heat down to medium and sprinkle over the flour, stirring well to evenly combine.
In a glass measuring cup or a small bowl, whisk together the tomato paste and Worcestershire sauce until smooth. Whisk in the beef broth and pour into the pan. Bring to a boil, stir through the frozen vegetables, and season with more salt and pepper, if needed. Reduce the heat to low and simmer while you prepare the topping.
Split the potatoes in half and scoop out most of the flesh, leaving some on the skin to hold the potato skins together. Transfer the flesh to a bowl along with the remaining ¼ cup of butter. Mash until very smooth, adding enough sour cream to give the potatoes a creamy consistency. Season with salt and, for easier topping, transfer the mashed potatoes to a piping bag fitted with a large star tip. Spoon the beef filling into the potato skins and pipe over the mashed potatoes. Scatter with the cheddar cheese and return to the oven for 10 to 15 minutes or until the cheese has melted and the top is lightly golden brown and crisp.
Allow the potatoes to cool slightly before serving.
