Put the potatoes on a baking sheet and roast until easily pierced with the tip of a sharp knife, about 1 hour. Let cool slightly. Do not turn the oven off.
Halve the potatoes lengthwise. Using a small spoon, gently scoop the potato flesh into a bowl, leaving about 1/2 inch on the skins. Set the skins aside. Reserve the potato flesh for another use.
Heat the olive oil in a large skillet over medium heat. Add the onions and jalapenos and cook, stirring occasionally, until soft, 5 to 7 minutes. Stir in the beef and continue to cook, stirring often, until the beef is cooked through.
Stir in the tomato paste, cumin, chili powder, garlic powder, salt and pepper to combine and cook 1 minute more. Add the heavy cream and cook, stirring, until incorporated and heated through, about 1 minute.
Spoon the beef mixture into the hollowed-out potato skins. Put the potato skins on a parchment-lined baking sheet. Sprinkle with the Cheddar and bake until the cheese is melted, about 3 minutes. Transfer to a platter and top with the green onions, tomatoes, avocados, cilantro and sour cream if using.