Step 1: Prepare the Sweet Potatoes

1. Preheat your oven to 400°F (200°C).
2. Wash and scrub the sweet potatoes thoroughly, then pierce them with a fork several times to allow steam to escape during baking.
3. Place the sweet potatoes on a baking sheet lined with parchment paper.
4. Bake for 45-60 minutes, or until the sweet potatoes are tender when pierced with a fork.

Step 2: Sauté the Filling

1. While the sweet potatoes are baking, heat olive oil in a large skillet over medium heat.
2. Add diced onions and sauté for 3-4 minutes until softened and translucent.
3. Add minced garlic and cook for another minute, stirring frequently to prevent burning.
4. Stir in the sliced mushrooms and cook for 5-7 minutes until they release their moisture and begin to brown.
5. Add the fresh spinach to the skillet and cook until wilted, about 2-3 minutes.
6. Stir in chopped rosemary, salt, black pepper, and red pepper flakes (if using). Mix well and cook for another minute.
7. Remove the skillet from heat and fold in the crumbled feta cheese and lemon zest.

Step 3: Assemble the Stuffed Sweet Potatoes

1. Once the sweet potatoes are done baking, remove them from the oven and let them cool for a few minutes.
2. Slice each sweet potato lengthwise down the middle, being careful not to cut all the way through.
3. Gently press the ends of the sweet potato together to open up the center, creating a pocket for the filling.
4. Spoon the mushroom, spinach, and feta mixture into each sweet potato, filling them generously.

Step 4: Garnish and Serve

1. Top each stuffed sweet potato with a sprinkle of chopped walnuts for added crunch (optional).
2. Drizzle a little olive oil over the top for extra flavor and shine.
3. Serve hot and enjoy your nutrient-packed stuffed sweet potatoes!