Preheat the oven to 400°F. Wash the sweet potatoes and pierce them several times with a fork. Place them on a baking sheet and bake for 45-60 minutes, or until tender.
While the sweet potatoes are baking, heat the olive oil in a large skillet over medium heat. Add the sliced mushrooms and cook for about 5-7 minutes, until they are browned and softened. Stir in the minced garlic and cook for an additional minute.
Add the fresh spinach to the skillet and cook until wilted, about 2-3 minutes. Season with salt, black pepper, and dried thyme. Remove from heat and let cool slightly.
In a mixing bowl, combine the cream cheese, Parmesan cheese, and half of the mozzarella cheese. Stir in the mushroom and spinach mixture until well combined.
Once the sweet potatoes are done baking, remove them from the oven and let them cool for a few minutes. Cut each sweet potato in half lengthwise and scoop out a small portion of the flesh to create space for the filling.
Fill each sweet potato half with the creamy mushroom and spinach mixture. Top with the remaining mozzarella cheese.
Return the stuffed sweet potatoes to the oven and bake for an additional 15 minutes, or until the cheese is melted and bubbly. Garnish with fresh parsley before serving.