Preheat oven to 450°F
Prick potatoes several times with a fork and place them directly on oven rack. Bake until tender, 50 to 60 minutes.
While potatoes are baking, heat the oil in a deep skillet over medium heat. Add the onion and cook until slightly translucent, about 10 minutes. Add the beef, stirring to break it up, and cook about 5 minutes, until browned.
Stir in the Worcestershire sauce, tomato paste, Parmesan cheese, 1/2 tsp salt, and pepper to taste. Cook another 5 minutes.
Remove potatoes from oven and set aside until cool enough to handle. Cut them in half lengthwise and carefully scoop out the flesh into a mixing bowl, leaving some behind to keep the shells intact. Mash potatoes lightly with a fork, then mix in the butter, yogurt, and 1/4 tsp salt. Set aside.
Fill each potato shell with about 2 Tbsp of the cooked hamburger mixture, then top with about 3 Tbsp of the potato mixture, mounding slightly. Top with a slice of American cheese and bake until melted, about 5 minutes. Serve immediately.
Per Serving 413 calories, 23 g fat (10 g saturated), 708 mg sodium, 57 mg cholesterol.
Quickie-side suggestion: Toss a bag of chopped romaine lettuce with 6 Tbsp olive oil, 2 Tbsp lemon juice, and parmesan cheese. Salad doesn't get much easier!
Ideas for picky eaters: This is a way fun dish to eat, so you may luck out. If not, just deconstruct it: Put a little mashed potato and beef on the side, and let him eat his cheese straight out of the wrapper.