Preheat the oven to 400°F. Pierce the sweet potatoes with a fork and place them on a baking sheet. 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 sliced mushrooms. Cook for another 5-7 minutes until the mushrooms are browned and have released their moisture.
Add the fresh spinach, salt, black pepper, and thyme to the skillet. Cook until the spinach is wilted, about 2-3 minutes.
Remove the skillet from heat and stir in the cream cheese and Parmesan cheese until well combined.
Once the sweet potatoes are done baking, let them cool slightly. Cut them in half lengthwise and scoop out a small portion of the flesh to make room for the filling.
Mix the scooped sweet potato flesh into the mushroom and spinach mixture.
Spoon the filling back into the sweet potato halves and 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 fresh parsley before serving.