Heat oven to 400°F. Prick top of each potato a few times with a fork, and bake directly on oven rack until easily pierced with a paring knife, about 45 minutes to 1 hour depending on size. Remove from oven; let cool slightly, about 10 minutes.
Using a sharp knife, cut potatoes in half lengthwise. Use a spoon to scoop flesh into a large bowl, leaving 1/4-inch border all around. Set shells aside. Combine flesh with remaining ingredients and mash with a potato masher until relatively smooth and combined. Refill shells with potato mixture.
Set on a baking sheet or baking dish lined with parchment paper. Cook until potato is soft and filling is lightly browned on top, 35 to 40 minutes.
Sprinkle with more chopped thyme and parmesan and serve.