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 olive oil in a large skillet over medium heat.
Add the diced onion and sauté for 3-4 minutes until translucent.
Stir in the minced garlic and cook for an additional minute.
Add the sliced mushrooms to the skillet and cook for 5-7 minutes until browned and tender.
Stir in the fresh spinach, dried thyme, salt, and black pepper. Cook until the spinach is wilted, about 2-3 minutes.
Remove the skillet from heat and let it cool slightly. Stir in the cream cheese and Parmesan cheese until well combined.
Once the sweet potatoes are done, remove them from the oven and let them cool for a few minutes.
Cut them in half lengthwise and scoop out a small portion of the flesh to create a pocket for the filling.
Fill each sweet potato half with the creamy mushroom and spinach mixture. Top with shredded mozzarella cheese.
Return the stuffed sweet potatoes to the oven and bake for an additional 10-15 minutes, or until the cheese is melted and bubbly.
Garnish with chopped fresh parsley if desired before serving.