Bake the potatoes. Preheat the oven to 400°F. Scrub the potatoes and prick them several times with a fork. Place directly on the oven rack or on a baking sheet and bake for 50–60 minutes, or until very tender.
Cook the bacon. While the potatoes are baking, cook the bacon until crisp. Transfer to a paper towel-lined plate, then crumble or chop into small pieces.
Cool the potatoes slightly. Let the baked potatoes cool just until they're easy to handle.
Scoop out the potato. Slice the potatoes in half and scoop the flesh into a large mixing bowl. Discard the skins or save them for another use.
Make the filling. Add the Greek yogurt, milk, butter, garlic powder, salt, and pepper to the potatoes. Mash until creamy, leaving a little texture if you prefer.
Add the mix-ins. Fold in 1 cup of the cheddar cheese, about two-thirds of the bacon, and half of the sliced green onions.
Transfer to a baking dish. Spread the potato mixture into a lightly greased 9x13-inch baking dish.
Add the toppings. Sprinkle the remaining cheddar cheese, bacon, and Parmesan over the top.
Bake again. Reduce the oven temperature to 375°F and bake for 20–25 minutes, or until heated through and the cheese is melted and bubbly.
Garnish and serve. Sprinkle with the remaining green onions before serving.