Preheat the oven to 400°F (200°C).

Using a fork, pierce each sweet potato several times. Place them directly on the middle rack of the oven. (Tip: Place a foil-lined sheet pan on the rack below the potatoes to catch any drips.)

Bake for 45-60 minutes, or until the sweet potatoes can be easily pierced by a fork.

Remove the sweet potatoes from the oven and let them cool until they can be handled.

While the sweet potatoes cool, use paper towels to press out as much liquid as possible from the thawed spinach.

In a medium bowl, combine the drained spinach, cilantro, olive oil, lemon juice, garlic, oregano, salt, pepper, feta cheese, and mushrooms; mix well.

Once the sweet potatoes have cooled, slice each potato lengthwise.

Scoop out most of the sweet potato, leaving the skins intact. Add the scooped-out sweet potato to the bowl with the spinach mixture; stir until well combined.

Stuff the sweet potato skins with the spinach and feta mixture.

Place the stuffed sweet potatoes on a foil-lined baking sheet and bake for an additional 8-10 minutes. Serve hot.