Roast the Sweet Potatoes
Preheat oven to 400°F. Wash and dry sweet potatoes, then pierce them a few times with a fork. Place on a parchment-lined baking sheet and bake for 45–60 minutes, until tender. Let cool for 10–15 minutes.
Make the Garlic Parmesan Dip
While the potatoes bake, combine yogurt, mayo, Parmesan, garlic, lemon juice, salt, and pepper in a bowl. Refrigerate to let flavors blend.
Cook the Filling
Heat oil or butter in a skillet over medium heat. Sauté onions until soft, then add garlic and mushrooms. Cook until mushrooms brown slightly. Stir in spinach, herbs, salt, and pepper until wilted. Remove from heat and mix in feta.
Prepare the Skins
Slice sweet potatoes in half lengthwise. Scoop out most of the flesh, leaving a ¼-inch border. Brush skins with olive oil and bake for 10 minutes at 400°F to crisp.
Assemble & Bake
Fill each skin with the mushroom-spinach mixture and top with shredded mozzarella. Bake for 10–15 minutes until cheese melts. For extra browning, broil for 2 minutes (watch closely!).
Serve
Garnish with fresh herbs and serve warm with the garlic Parmesan dip.